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City of College Station
POST OFFICE BOX 9960 1 l 01 TEXAS A VENUE
COLLEGE ST A TION, TEXAS 77840-2499
February 14, 1986
INTEROFFICE MEMORANDUM
TO: David J. Pullen, City Engineer
FROM:i{)· John R. Black, Traffic Engineer
RE: Dr i veway Access Proposed by Dalsan Properties on Texas
Avenue South of SH 30
I have reviewed the College Station Retail Center site plan with
access on Texas Avenue limited to one drive with a right-turn
deceleration lane. This revision includes a 125 ft. storage lane
with a 40 ft. taper versus the 75 ft. storage, 125 ft. taper
recommended in my correspondence to you dated January 10, 1986.
I have reviewed the 1985 ITE "Guidelines for Driveway Design &
Location" along with the 1984 AASHTO "Geometric Design of
Highways and Streets" in an attempt to find a warranting
procedure for right-turn lanes. I have concluded that except for
very high traffic generators (regional mall, parking garage,
etc.) right-turn deceleration lanes should only be considered for
street intersections where peak hour volumes in the outside lane
of the major street exceeds 900 vehicles/hour with 10 percent or
more right-turns.
I did not recommend the right-turn lane into College Station
Retail Center based on these guidelines because traffic generated
by this site will never be great enough to justify a deceleration
lane. This suggestion was made in an attempt to move southbound
right-turning traffic on Texas Avenue out of the thru lanes to
limit exposure to rear-end accidents. This section of Texas
Avenue is complicated by the weaving area created by the traffic
signal at SH 30. The 75 ft. storage and 125 ft. taper distances
recommended in my letter dated January 10 are minimum standards
from the State Department of Highways Operations and Procedures
Manual (see attached). The revised plat from Dalsan Properties
does not provide a long enough taper distance and the island
design does not reflect current AASHTO standards. Because our
intent is to move turning traffic out of the thru lanes as
quickly as possible, I believe we should reject an inadequate
deceleration design in favor of a strategy to reduce the exposure
time of the right-turn movement (see ITE Guidelines, P. 19;
Access Management and Driveway Design, P. 6-7).
David J. Pullen
Driveway Access/Dalsan Properties
Page 2
Therefore, I suggest that we approve the single access drive
without a deceleration lane using the AASHTO guidelines. This
design calls for a simple curve radius {35 ft.) with taper {6 ft.
offset, 10:1 taper rate), no approach island and a reduction in
the divisional island separating ingress and egress. There is
not enough space between the parking lot and Texas Avenue to
construct a right-turn island designed to AASHTO guidelines {p.
768). However, the divisional island separating ingress and
egress will insure that the space needed by a right-turning
vehicle entering this site is not blocked by an exiting vehicle.
This divisional island should be reduced 25 ft. as shown to
provide for an occasional delivery truck {WB-40). The design
provided takes into consideration the high volume and weaving
characteristics of this section of Texas Avenue to maximize safe
ingress through current geometric design policies.
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City of College Station
POST OFFICE BOX 9960 l I 0 I TEXAS A VENUE
COLLEGE ST A T I ON, TEXAS 77840-2499
January 10, 1986 ·'
INTEROFFICE MEMORANDUM
TO:
FROM: (d-0 ·
SUBJECT:
David J. Pullen, City Engineer
John R. Black, Traffic Engineer
Driveway Access Proposed by Dalsan Properties on Texas
Avenue just South of SH 30
I have reviewed the site plan for a propo~ed retail center from Dalsan
Properties · and understand the owner• s concern for driveway access:.
Please cons-.ider ·the attached which replaces the two drives proposed
with a single -drive located as far as possible from the traffic signal
at SH 30. "I:_he 125 foot taper, 75 foot storage lane . and 50 foot .radius
shown should provide better ingress to this site for southbound traffic
since the driveway intersects Texas Avenue at 60 rather than 90 degrees.
I do not believe it is practical to construct two one-way drives in
this 200 foot section and that the available frontage is best utilized
to improve tight-turn access. ·
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10
a one-w ay driveway, t he proper ra-
dius for the side not used for right
turn ent ry (or exit) is established
by the swept path needs of a ve-
hicle entering by a left turn from
the far side of the street (or exiting
by a left turn onto the far side). Ex-
cept for very narrow streets or for
large vehicles, the "off-side" radii
may be small.
Since parking may be prohib-
ited in the future along any major
thoroughfare, and a curb lane ad-
jacent to a given driveway may be
clear at times on any street, it is
good practice to design driveways
(other than single-family residen-
tial driveways on local streets or
low-volume collectors) for entry
from the curb without encroach-
ment onto adjacent lanes or be-
yond the centerline.
The radius used at a given
driveway is meaningful only when
related to the width of throat. This
throat is basically a point of n ar-
rowest controlled width. When
the distance between curb line
and right-of-way is equal to or
greater than the design radius,
the throat width may conveni-
ently be measured along either
the property line or the end of the
radius. In many cases, especially
in urban areas, the proper radius
will be greater than the distance
between curb line and property
line. In such cases, if a raised bar-
rier curbing extends into private
property, the throat width may ap-
propriately be measured at the
end of the radius even though this
may be on private property.
It iD recommended that, as a
general rule, the w idths of two-
way driveways · be measured par-
allel to the roadway. One-way
driveways may be measured at
right angles to the driveway.
When a center channelizing is-
land is used in a two-way drive-
way to restrict entries to right
turns in and right turns out, it is
also appropriate to measure the
width separately and at right an-
gles between the curbing of the
channelizing island and the drive-
way curb return. In this type of de-
INSTITUTE OF TRANSPORTATION ENGINEERS
sign, radii and total width of drive-
w ay at the t h roat a re usually
somewhat g reate r than for a two-
way driveway without a channe-
lizing isla nd due to the need for
lateral clearance between faces of
the barrier curbs.
The design guidelines for the
minimum width of driveways,
measured at the throat or at an-
other control point, range from 10
to 20 feet (3 to 6 m). For commer-
cial or industrial driveways, this
minimum width is based on one-
way operation. Widths range from
30 to 40 feet (9 to 12 m), depending
on type of land use served, for
two-way operation. The use of
channelizing islands in any of
these driveways, however, should
automatically produce variations
for such additional widths as nec-
essary to assure efficient and safe
traffic movements. In general, me-
dians separating driveway move-
ments are seldom needed except
to channelize turns (such as
shown in Figure 4 and the lower
Figure 4. One-w ay angle driveways, left
turns prohibited.
part of Figure 5) or for high volume
4-lane driveways (see upper part
of Figure 5). The center median (if
curbed) usually varie s from 4 feet
(1.3 m) to 12 feet (4 m) in width.10
The wider medians a re more at-
tractive if landscaped and often
contain a site identification sign.
Figure 5. Ninety-degree highway design
allowing good entry and exit speeds; not
for use in high pedestrian activity areas.
Anything placed in the median
(even a Keep Right sign) should be
setback and/or so maintained as
to not restrict visibility in the
driver eye height range from 3% to
81/i feet (1to2.5 m) above grade.
Where public sidewalks abut
the curb in an urban area, it may
be difficult to make the edge of the
driveway visually apparent if the
sidewalk is warped down into the
driveway rather than using a step-
d own curb along the edge of the
drive. While most driveways will
function satisfactorily with
warped sidewalks, thus avoiding
pedestrian inconvenience, use of
step-down curbs warrants con-
sideration for special circumstan-
ces, since curbs have the impor-
tant secondary advantage of
notifying the pedestrian that he is
in a zone of conflict.
If step-down curbs are used,
sidewalk ramps for use by persons
in wheelchairs and other physi-
cally handicappe d persons are
often used and may be mandated
by state or federal regulations.
A recent, statewide study of
driveway accidents in Texas by
Rogness and Richards37 found
only 1 p e rcent to involve pedes-
trians. This confirmed an earlier
study by Box6 in Skokie, Illinois
which found about 1 percent to in-
volve pedestrians. These data
GUIDELINES FOR DRIVEWAY DESIGNS & LOCATIONS 19
1800 I -4 ~ -i f --· I ~ . -~ --- --, --r :-~-1 1600
\\ R\' ~ --,_ ----j . r ·--, Grade, unsignalized intersections 1----~----~ ,_ l ' I
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0 50 100 150 200 250 300 350 400 450 500 550 600
VL: LEFT TURNING VOLUME (VPH)
Figure 10. Warrants for left-tum storage Janes on four-Jane, at-grade unsignalized highways. The section on graph lying between
"undivided" and "divided" (V L = 25 to 55 vph for a V0 level of 200 vph) relates to a w arrant for a one-space length as provided by an
ordinary opening in a median about 20 feet (6 m) wide. Source: Reference 15, Figure 1.
that upstream and downstream
factors affecting a driveway loca-
tion be considered in each in-
stance.
As discussed under successive
entrances, the entry movement to
a series of driveways serving in-
terconnected or common parking
areas tends to be heavily concen-
trated at the first driveway in the
series. Thus, deceleration lanes
for right turns may be needed only
at the first one or two driveways
serving a given approach to a ma-
jor facility. The value in a deceler-
ation lane and the length of lane
required is a function of the right-
tuming volume into the driveway,
the volume in the curb lane, and
the s pee d of entry allowable by
the driveway's geometric design.
Driveways with relatively high-
speed entries, such as the ones
c-~~
shown in Figure 5, may require no
d eceleration lane fo r e ven high
volumes.
Cottrell presents a set of guide-
lines for right turn lane warrants
and tape rs, applying to rural
roads.55
Traffic signal control of high vol-
ume driveways is commonly ac-
cepted in most jurisdictions. The
control is need ed primarily to facil-
itate out bound left turn move-
ments, when heavy volumes of
through traffic cannot be accom-
modated simultaneously. If the
outbound left-turn movement is
low, the two-way flow on the m a-
jor route m ust be stopped by the
s ign a l for only a s h ort p e riod.
However, efficient s ignal opera-
tion under such condition s re-
quires s e parate sensing of the
driveway's right-and left-turn
traffic lanes. Unless these lanes
are separated and are of sufficient
width, t his may not be feasible. If
separate sensing is not used, ex-
cessive green time will be re-
quired to the detriment of
through-traffic flow on the major
route.
As noted under Median Cuts,
driveway traffic signals within 112
m ile (800 m) of another signalized
intersection should be coordi-
nated. The 112 signal concept also
w arrants consideration, as previ-
ously noted. 0· '0 ·53
Because of the complexitie s and
costs (both public and private) of
prov iding access to major tra ffic
generators, competent traffic stu-
dies should precede issuance of
access permits. The intimate re-
lationship between driveway lo-
cations and interior traffic circula-
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Storage Lengt~
R 1 =2R 2 (approx.)
Tangent Length = (lf3 To 1/2) (taper length)
Taper length and storage length from table.
Taper Length
LENGTHS OF SP EED CHANGE LA NES-URBAN STREETS
Design Ta per Length Storage Length (feet) 31J -
Speed (feet) Signalized Non -Signalized
(mph) minimum min. des. min. des.
30 80 *see '170 50 170
40 125 note 245 75 245
50 180 below 320 100 320
Based on desiqn hour vo.lume, Storage Length= (2) (overage no. of vehicles stored per signal cyc le)
Total length of speed change lone = Storage Length+ Taper Length
Applicable to speed change lanes to accommodate left or U-turns at median openings or
applies also to speed chonge lanes for right turns where desired .
i nterse ction~ ~
* * * * Block spacing may dictate lesser values
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Driveway Throat Width l w = o_i I I Available Entry Width Study 2 w = w ( •
0
0 5 10 15 20 25 30
Curb Return Radius. Feet
SOURCE: Ref. (2)
FIGURE 6-4 THE INFLUENCE OF DRI VEWA Y WIDTH
AND CURB RETURN RADIUS ON DRIVEWAY ENTRY SPEED.
6-6
vehicle stopped in the throat has a greater effect on the speed and pat h
of the ; ·ght front wheel than explained by the reduction in t he available
throat wi dth only . This u n doubtedly results from the driver's desire
(requ irement) to maintain a g r eater cl earance between his or her vehicle
a nd a stopped vehicle than an edge of the driveway .
Nevertheless , a simple review of Fi g ure 6-4 clearly indicates that t he
speed of a vehicle entering a driveway is very slow from all reasonable
combinatio n ~ of throat width and curb return radii . Even excessive radii
(25 to 30 feet) and throat width (30 to 35 feet) result in speeds of only 12
to 13 miles per hour. Consequently, a vehicle making a driveway maneu-
v er is traveling at slow speeds at a considerable distance upst ream from
the d r iveway whi ch t he driver plans to ente r (see Figures 6-5 and 6-6).
As a result, a speed differential of 10 mph or more occurs at least 250 feet
upstream from the driveway for off-peak arterial street speeds of 40 to 45
mph.
The fact that excess ive speed di ffe rentials are created a considerable
distance u pstream from the point at which t he driveway maneuve r is made
probably r esults in an under-reporting of driveway related accidents on
accident reports. It also shows that t urn la nes are needed in order to
achieve acceptable speed differentials between d ri veway traffic and through
vehicles on arterial stree ts.
Dr iveway designs employing a taper on the upstream side of the
driveway were also studied (see Figure 7). The speed of the vehicles in
the d riveway maneuver was not different from that for the simple curb
retu rn radius. However, the dispersion of the path of the right f ront
wheel in the driveway throat is reduced . More importantly, the taper
results in a reduction in exposure time (the time which the turning vehicle
is b locking the through traffic lane). Comparison of the vehicle trajec-
to r ies for the direct and spiral tapers shown in Figure 6-8 indicates that
t he spira l produces a more consistent trajectory (less dispersion) and
encour ;i~;es the d r iver to follow the cur b line and thus clear the through
tr·affic l2ne earlier than t he direct taper design.
A ot::si gn u sing a relati ve ly large radius on the upstream curb return
and a small radius on the downstream side of th e d r iveway (see Figure
6 -9) wa s used to evaluate the possible effect on driver behavior. Com-
parison of the trajectory pattern (see Figure 6-10) with that of the same
6-7
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30 I Driveway
W=30'
R=3 0'
25
20
15
10
5
0 50
4
100
Speeds at all the other
driveways were within these houndaries.
including the following des igns:
\~=30 '; R= O'
W=2 5'; R= 5'
W=35 '; R= 5 '
'tJ=20 I ; R= 10 I
W=30'; R=lO'
W=25 I ; R=ZO I 9
W=35 I; R=20 I
W=20 I ; R=30 I
Distance (Feet) Upstream From Drive
SOURC E: Ref. (2)
FIGUR E 6-5 SUMMARY OF VEHICLE SPEED DATA WHEN THER E IS
NO EX IT ING VEH ICLE STOPPED IN THE DR I VEWAY THROAT.
6-8
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30
25
15
10
5
0
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W'=20'
R =20'
50 100
W'=lO'
R = 5'
41~
Speeds at all the other Study 2
driveways were within these boundaries,
including the following designs:
w I =20 f;
W'=l0 1
;
W'=l5';
150
R=20'
R=20'
R= 51
200 250
Distance (Feet) Upstream From Driveway
SOURCE: Ref. (2)
FIGURE 6-6 SUMMARY OF VEHIC LE SPEED DATA WHEN AN
EXITING VEHICLE IS STOPPED IN THE DRIVEWAY THROAT.
6-9
24 ' I• go• _____ ,.I
Direct Taper R=lO R=lO'
30'
Direct Taper Approach Treatment
t 24' _....
~ 90• ____ ,..I
Spiral Taper R=lO'
30'
Spiral Taper Approach Treatment
SOURCE: Ref. (2)
FIGURE 6-7 OFF-SET TAPER APPROACH TREATMENTS EVALUATED.
6-10
Taper Lf"n9tf\ • 90'
Ofh~t • 9'
,__ __ ·~· __ __.,.
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Direct Taper Approach Trea tmen t
h pr"r tf"nqtti " 90'
Oftut • q'
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Spiral Taper Approach Treatmen t
SOURCE: Ref. (2)
,--
11'
,--
12'
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FIGURE 6-8 AVERAGE PATH OF THE TEST VEHICLE1S RIGHT
FRONT TIRE DURING RIGHT TURN ENTRY MANEUVERS.
6 -11
At-Grade Intersec1io11::, 727
plane at the intersection, the particular plane depending on direction
of drainage and other conditions. Changes from one cross slope to
another should be gradual. Intersections with a minor road crossing a
multilane divided highway with a narrow median and superelevated
curve should be avoided whenever possible because of the difficulty in
adjusting grades to provide a suitable crossing. Gradelines of separate
turning roadways should be designed to fit the cross slopes and
longitudinal grades of the intersection legs.
As a rule, the alinement and grades are subject to greater restriction
at or near intersecting roads than on the open road. Their combination
at or near the intersection must produce traffic lanes that are clearly
visible to the operators at all times and definitely understandable for
any desired direction of travel, free from sudden appearance of poten-
tial hazards, and consistent with the portions of the highway just
traveled.
The combination of vertical and horizontal curvature should allow
adequate sight distance at an intersection. A sharp horizontal curve
following a crest vertical curve is very undesirable in the intersection
area.
INTERSECTION CURVES
Widths for Turning Roadways
at Intersections
The pavement and roadway widths of turning roadways at intersec-
tions are governed by the volumes of turning traffic and the types of
vehicles to be accommodated and may be designed for one-way or
two-way operation, depending on the geometric pattern of the intersec-
tion. Widths determined for turning roadways are also applied on
through roadways within an intersection, such as between channelizing
islands. The turning radii and the pavement cross slopes are functions
of design speed and the type of vehicles. For an in-depth study of the
criteria involved, see Chapter III .
Oearance Outside Pavement Edges
MlnJmum Designs for Sharpest Tums
Where it is necessary to provide for turning vehicles within mm1-
mum space, as at unchannelized intersections, the minimum turning
728 AASHTO-Geometric Design of Highways and Streets
paths of the design vehicles apply. Sharpest turns of these vehicles are
shown in Figures Il-1 through Il-10, and the paths of the outer front "
wheel,. the inner rear wheel, and the front overhang are shown. The
paths indicated, which are slightly greater than the minimum paths of
nearly all vehicles in each class, are the minimums attainable at
speeds less than 10 mph, and consequently offer some leeway in driver
behavior.· Layouts patterned to fit these paths of design vehicles are' ·
considered. satisfactory as minimum designs where turning speeds of ·.
10 mph or less are appropriate.
In the design of the edge of pavement for the minimum path of a
given design vehicle, it is assumed that the vehicle is properly posi:' '
tioned within the traffic lane at the beginning and end of the turn, i.e., "
2 ft from the edge of pavement on the tangents approaching and . '
leaving the intersection cu.rve. Curve designs for edge of pavemept
conforming to this assum.ption are shown in Figures IX-2 through IX-6.
They closely fit the inner wheel paths of the various design vehic:Ies,
clearing them by 2 ft or more throughout most of the turn, and at no
point by less than 9 in. Differences in the inner path of vehicles
turning left and right are not sufficient to be significant in design.
Although not shown separately in Figures IX-2 through IX-6, these
edge designs also apply for left-turn layouts, such as a left tum to ·.!
leave a divided highway at very low spe~d.
At an intersection with low right-turn volumes, the designer may
determine that the need for a deceleration and right-turn lane is not".
warranted. In this instance, the composition of the shoulder may be
improved for greater load capacities to permit right-turn vehicles.'l o •
utilize the shoulder .. with no d~trirnental effects to the shoulder. Wh~re~
right-turn traffic volumes are high, a shoulder surfacing similar io"'tbe'
main line p~vement sh~uld be used to .provide a location for decfle~-? ..
ing vehicles which desire to make a right turn to sto~e in dJ!iir!&'
turning operation. Full or partial shoulders should be considef~d
where deceleration lanes for right turns are utilized ..
r
Passenger Vehicles
,
Three minimum designs for the inner edge of pavement for a 90~
right turn to accommodate the P design vehicle are shown in Figure
IX-2. A 25-ft radius on the inner edge of pavement (the solid line 1!1
Figure IX-2A) is the sharpest simple arc that clears the inner whe~J
path and does so by about 9 in. near the end of the arc. A simple cunle, . .,.,
of 30-ft radius, shown dotted in the same figure, provides a 1.2-ft '
At-Gradt
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A t-Grade Intersections
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o' '<.,~ x' ,._t>, ~ ,.,,~" / "\. 4,o
MIN1My11. SIMPLE.VURVE.
/WITH TAPER,
20' kA01us-,OFFSET 2.5'
-8-"
r· or s 1Gr,
\'f f11 .~L ; I LTh
P DESIG~;
V:CHICLE PATH
P DE::IGN
VE HICLE PATH
Figure IX-2. M inimum designs for passenger vehicles.
729
. j
I !
730 AASHTO-Geometric Design of Highways and Streets
"" ~-3-CENTEREO COMPOUND CURVE ,
""" 120· -JO'~t20' RADII, Of:FSET 2 .0' f>.'I; v -c-v
Figure IX-3. Minimum designs for single unit trucks
and buses.
At-(
At-Grade Intersections
WB 40 SEMITRAILER COWBINATION
3-CENTERED COMPOUND CURVE
120' -40' -200' RAD! I
OFFSET Z AND 6'
WB -40 SEMITRAILER CX>MBINATION
3-CENTEREO CCMPOUND CURVE
120' -4o' -12d, OFFSET 5'
731
WB-40 DESIGN
\'[·1:c_, E PATH
W B -40 DESIGN
VEHICLE PATH
Figure IX-4. Minimum designs for semitrailer combinations
(WB-40 design vehicle path).
732 AASHTO-Geometric Design of Highways and Streets
WB-50 DESIGN
VEHICLE A:\TH
MINIMUM SIMPLE CURVE
WITH TAPER
60' RADIUS , OFFSET 4 0'
Figure IX-5. Minimum designs for semitrailer combinations
(We:50 design vehicle path).
~· &4 clearance at the ends of the curve but has a clearance -0f about SA ft at -' . the middle of the curve. On a radius of more than 30'ft .mos~ driver~.,
would naturally use a turning_radius flatter than the rninimum:for their"
passenger-vehicles and more or less follow the edge of pavement. ~
The edge_ ~esign shown in Figure ~-2C is a practical _equivalent to,,:; a,.~,,.,'.,:, .. ,:
curve trans1t1onal for most or all of its length. It cons~sts of a three•-1-
centered curve with radii of 100, 20, and 100 ft, the center of'the
middle curve being located ~~.S ft from the extensio'a of the tangent
edges (which measurement includes a 2.5-ft offset). Thi~ design in:::..
volves little extra pavement as compared with the simple curve of 30:ft -
radius, the pavement areas in a single quadrant between the tangen(-
edges of pavement produced being 24 yd2 for the edge design and 21.,S
yd2 for the -simple curve. However, the edge design is preferr~~
because it fits the design vehicle path more closely. A design that
closely approximates the three-centered curve layout in the field is
shown in Figure IX-2B. It consists of a simple offset curve and'·
connecting tapers.
At-(
F
Streets
ons
5.4 ft at
drivers
for their
~nt.
.ent to a
a three·
· of the .• .. ;_-·
tangent
sign.in-""~
of 30-ft • -. ~-
:ve
A i-Grade lnter!>ecrions
W8 -50 DL SIGN
VEHICt l ··ATH
WB -50 SEMITRAJLER co.eNATION
3 CENTERED CCMUW a.RYE ,
120'-40'-200' RADI. CFFSET 2' tit-0 !()'
v.13 -50 DESIGN
VEHICLE PATH
WB -50 SEMITRAILER CXM31NATION
3 CENTERED CCMl(X..NJ a...Rv'E ;
ec>'-60'-ec>', OFFSET 6'
733
Figure IX-6. Minimum designs for semitrailer combinations
(WB-50 design vehicle path).
734 AASHTO-Geometric Design of Highways and Streets
Single-Unit Trucks and Buses
Minimum designs for the inner edge of pavement for a 90° right
turn to accommodate the SU design vehicle are shown in Figure IX-3.
A 50-ft radius on the inner edge of pavement (the solid line shown in
Figure IX-3A) is the sharpest simple arc that accommodates this
vehicle without encroachment on adjacent lanes. Toward the end of the .
tum, however, the inner wheel path closely approaches the edge of ' ..
pavement. A simple curve of 55-ft radius, shown dotted in Figure ·
IX-3A, allows for slightly more clearance at the far end of the intersec-
tion curve. Inner-edge radii of 60 ft or more permit the SU vehicle to
turn on a radius greater than the minimum.
The design shown in Figure IX-3C is a practical equivalent to a curve
transitional for most or all of its length. It consists of a three-centered
compound curve with radii of 120, 40, and 120 ft with the center of the
middle curve located 42 ft from the extension of the tangent edges
(with a 2-ft offset). In an operational sense this design is much
preferred over the one with the minimum simple arc because it better
fits the minimum path of the inner rear wheel. Because the resulting
areas between the edges of pavement are 53 yd2 for the compound
curve design and 60 yd2 for the 50-ft radius, single-arc design, the
former design requires less pavement. A simpler tapered design that
closely follows the three-centered layout is shown in Figure IX-3B.
In any design permitting the SU design truck to turn on its minimum
path without swinging wide, the front overhang swings out 12 ft from
the edge of tangent pavement on the far end of the tum, and the
vehicle thereby fully occupies a 12-ft lane on the crossroad. With 10-or
11-ft lanes, t~e vehicle would encroach on adjacent lanes. To preclude
encroachment with lanes less than 12 ft wide, edge-of-pavement radii
larger than the minimum indicated would have to be used.
Semitrailer Combinations
For the design semitrailer combinations· it is not practical to fj.t : ·
simple circular arcs to their minimum design paths. However, where .
traffic lanes are 12 ft wide, such vehicles can turn without encroach· _, . ~
ment on adjacent lanes when the radius of a simple curve on the inner.' · . • . . .. -:.t-
edge of pavemen~ is approximately 75 ft for the WB-40 ~nit a~~ 9~ ft ·:~
for the WB-50 umt. Such turns would be made on a turnmg radius (of ·-· ·•·
the outer front wheel) greater than the minimum shown for these ,
vehicles. To fit the edge of pavement closely to the minimum path of ,.·
1Jw
mc1
VV H
the
the
line
tion
effr
ma~
s
Joni
disc
des
tor
thei
on!~
lay<
ght
t-3.
tin
this
the
J>t
UJe'
ICC--~ ~""' • : to
rve
red
the
~es
1ch
ter
1ng
ind
the
iat
1m
>m
he
or
e
dii
fit
,e
b~
pr
ft .
~f
se
of
~
Ar-Grade lnrt'r.H:'Nion.\ 735
t tic design semitrailer combinations, it is ncces~ary to use an asym-
metrical arrangement of three-centered compound curves. For the
WB-40 design vehicle these cu rves have radii of 120, 40, and 200 ft,
the arc of the middle curve being offset 2 and 6 ft fro m the extension of
the tangent edges on the approach and exit sides (shown as the solid
line in Figure IX-4), respectively. A simple curve with taper combina-
tion is shown in Figure IX-5 for the WB-50 vehicle. Although not as
efficient in the use of pavement as the asymmetrical curve layout, it
may be a preferred design because of its ease of construction.
Some States legally allow semitrailer combinations 60 ft or more
long. The 60-ft combination is designated as a WB-55 vehicle. As
discussed in Chapter II, the WB-50 vehicle is the largest practical
design vehicle. Because the WB-55 vehicle is generally confined by law
to high-type facilities that can accommodate their tracking because of
their generous radii, values in this chapter regarding this vehicle are
only given in Tables IX-1 and IX-2. These values may be used in the
layout of special facilities or for crossroad turning movements.
Simple Curve Radius
Angle Simple With Taper
of Turn Curve Radius Offset Taper
(degrees) Design Vehicle Radius (ft) (ft) (ft:ft)
30 p 60
SU 100
W B-40 150
W B-50 200
WB-55 275
45 p 50
SU 75
W B-40 120
WB-50 120 2.0 15:1
WB-55 140 2.0 15:1
60 p 40
SU 60
WB-40 90
WB-50 95 3.0 15:1
WB-55 110 3.0 15:1
Table IX-1. Minimum ed ge-of-pavement designs for
turns at intersections.
736 AASHTO-Geometric Design of Highways and Streers At-Crud
Simple Curve Radius
Angle Simple With Taper
Angle• of Turn Curve Radius Offset Taper Turn (degrees) Design Vehicle Radius (ft) (ft) (ft:ft) I degree
75 p 35 25 2.0 10:1
30 SU 55 45 2.0 10:1 WB-40 ..... 60 2.0 15:1 WB-50 65 3.0 15:1 WB-55 90 3.0 15:1
90 p 30 20 2.5 10:1 45 SU 50 40 2.0 10:1 WB-40 45 4.0 10:1 WB-50 60 4.0 15:1 WB-55 75 4.0 15:1
105 p 20 2:5 8:1 60 SU 35 3.0 10:1 WB-40 40 4.0 10:1 WB-50 55 4.0 15:1 WB-55 65 5.0 15:1
120 p 20 2.0 10:1 75 SU 30 3.0 10:1 WB-40 35 5.0 8:1 WB-50 45 4.0 15:1 WB-55 50 6.0 15:1
135 p 20 1.5 15:1 90 SU 30 4.0 8:1 WB-40 30 8.0 6:1 WB-50 40 6.0 10:1 WB-55 45 7.0 10:1
105 150 p 18 2.0 10:1 SU 30 4.0 8:1 WB-40 30 6.0 8:1 WB-50 35 7.0 6:1 WB-55 40 7.0 8:1
120 180 p 15 0.5 20:1 SU 30 1.5 10:1 WB-40 20 9.5 5:1 WB-50 25 9.5 5:1 WB-55 25 14.5 5:1
Tt
Table IX-1. Continued.
f ·'Ir• "· • A t-Grade /111er:,ec1io11s 737
-UI! 3-Centered Compound 3-Cent ered Compound
Angle of Curve Symmetric Curve Aaymmetric
aper Turn Design Radii Offaet Radii Offset
ft:ftl (degrees) Vehicle (ft) (ft) (ft) (ft)
0:1 30 p 0:1 SU 5:1 WB -40
5:1 WB-50
5:~ WB-55
45 p
SU
WB-40
WB-50 200-100-200 3.0
WB-55 200-120-200 3.0 140-100-300 1.0-1.5
8:1 60 p
0:1 SU
WB-40 0:1 WB-50 200-75-200 5.5 200-75-275 2.0· 6.0 5:1 WB-55 200-85-200 5.0 120-80-240 1.0-7.5
5:1
0:1 75 p 100-25-100 2.0
0:1 SU 120-45-120 2.0
8:1 WB-40 120-45-120 5.0 120-45-200 2.0-6.5
WB-50 150-50-150 6.0 150-50-225 2.0-10.0 5:1 WB-55 200-70-200 7.0 120-60-200 2.0-10.0 5:1
5:1 90 p 100· 20-100 2.5 °°\
8:1 SU 120· 40-120 2.0
WB-40 120· 40-120 5.0 120-40-200 2.0· 6.0 6:1 WB-50 180· 40-180 6.0 120· 40-200 2.0-10.0 0:1 WB-55 200-65-200 7.0 100· 55-260 2.0-10.0
0:1
105 p 100-20-100 2.5 0;1 SU 100· 35-100 3.0 B:l WB-40 100· 35-100 5.0 100· 55-200 2.0-8.0
B:1 WB-50 180· 45-180 8.0 150-40-210 2.0-10.0
6:1 WB-55 240-50-240 8.0 100-45-500 4.0-10.0
B:1
120 p 100· 20-100 2.0
():1 SU 100-30-100 3.0
0;1 WB-40 120· 30-120 6.0 100-30-180 2.0· 9.0
WB-50 180-40-180 8.5 150· 35-220 2.0-12.0 5:1 WB-55 400-40-400 10.0 100-35-500 6.0·15.0
5:1
5:1
~· ~ . ?:.:-.~ Table IX-2. Minimum edge-of-pavement designs for
turns at intersections.
738 AASHTO-Geometric Design of Highway -and Streers
Angle of
Turn Oeaign
(degrees I Vehicle
135 p
SU
WB-40
WB-50
WB-55
150 p
SU
WB-40
WB-50
WB-55
180 p
SU
WB-40
WB-50
WB-55
3-Centered Compound
Curve
Radii
(ft}
100-20-100
100-30-100
120-30-120
160-35-160
300-35-300
75-85-75
100-30-100
100-30-100
160-35-160
300-40-300
50-15-50
100-30-100
100-20-100
130-25-130
300-25-300
Symmetric
Offset
(ft)
1.5
4.0
6.5
9.0
10.0
2.0
4.0
6.0
7.0
9.5
0.5
1.5
9.5
9.5
14.6
3-Centered Compound
Curve
Radii
(ft}
100-25-180
13:1-30-186
100-35-400
90-25-160
120-30-180
120-30-400
85-20--150
100· 25-180
180-35-200
Aaymm&tric
Offatrt
(ft}
3.0-13.0
3.0-14.0 3.o-15.o
1.0-12.0
3.0-14.0
7.0-16.0
6.0-13.0
6.0-13.0
10.0 12.0
Table IX-2. Continued.
Choice of Minimum Design
for Specific Conditions
The designs in Figures IX-2 to IX-6 are those that fit the sharpest
turns of the different design vehicles. Combinations of curves with
radii other than those shown may be used to produce similar and
satisfactory results. Of particular concern is the choice of general
design (Figures IX-3, 'IX-4, IX-5, and IX-6) for a specific intersection or
turning movement where it is desirable or necessary to keep the
intersection area to a minimum. The selection of any one of the
designs depends on the types and sizes of vehicles that will be turning
and to what extent they should be accommodated. In turn,. these
elements may depend on other factors such as the type, character, and
location of the intersecting roads and traffic volumes thereon, the
number and frequency of the larger units involved in turning move·
ments, and the effect of these larger vehicles -0n other traffic. For
example, if turning traffic is nearly all passenger vehicles, it is
wasteful to design for large trucks, provided that it remains possible
for an occasional large truck to turn by swinging wide and encroaching
on other traffic l_anes without disrupting traffic too much .
nd
1Ctoc; ••t
I
13.0
,4.0
15.0
12.0
14.0
16.0
·13.0
·13.0
·12.0
sharpest
ves with
nilar and
f gen ml
;ection or
keep the
1e of the
1e turning
rn, these
tcter, and
reon, the
ng move·
affic. For
:!es, it is
s possible
1croaching
_1~ f· Gnulr l nrersrc1 •' \ 739
It is necessary. therefore, for the designer to analyze the likely paths
;rnd encroachments that result when a turn is made by vehicles larger
; han those for which 1he design is made.
From the anal~ ..,j-, of these maneuvers and corresponding paths,
together with other pertinent data, the appropriate type of minimum
desig n can be selected. Applications of minimum designs for turning
movements are numerous, even in rural areas. Minimum designs may
be appropriate where speeds are generally very low, property values
are high, and turning movements are •low in volume. The selection of
the design vehicle for minimum design (Figures IX-2 through IX-6) will
depend on the judgment of the designer after all the conditions have
been analyzed and rhe effect of the operation of larger vehicles has
been evaluated. As a general summary, three minimum edge-of-
pavement designs for turns may be considered for the following
conditions:
P design (Figu re IX-2). This design is used at intersections in
conjunction with parkways where absolute minimum turns are stipu-
lated at local road intersections on major highways where turns are
made only occasionally and at intersections of two minor highways
carrying low volumes. In most of these instances, however, the SU
truck design (Figure IX-3) is preferred where conditions permit.
SU design (Figure IX-3). Generally this design is the recommended
minimum for all rural highways for conditions other than those de-
scribed. Turning movements for urban conditions are discussed in a
separate section of this chapter. Important turning movements on
major highways, particularly those involving a large percentage of
trucks, should be designed preferably with even larger radii, speed-
change lanes, or both.
Semitrailer combination designs (Figures IX-4, IX-5, and IX-6).
These designs should be used where truck combinations approximat-
ing this size of design vehicles will tum repeatedly. Where designs for
such vehicles are warranted,the simpler symmetrical arrangements of
three-centered compound curves (shown as the dotted lines in Figure's
IX-4. IX-5, and IX-6) generally are preferred if smaller vehicles make
up a sizable percentage of the turning volume. Because these designs,
particularly when used in two or more quadrants of an at-grade
intersection. prodtJce large paved areas that may be difficult to control,
it is usually desirable to channelize them, for which somewhat larger
radii arc needed. (See the section "Divisional Islands at Intersections"
in this chapter.)
712 AASHTO-Gcometric De.sign <JJ fi,~11"· .... 1: ;·,J Strlefs
types of turning vehicles. Minimum turning paths for pas~enger vehi-
cles, single-unit trucks, buses, semitrailer combinations, and full-
trailer combinations are included in Chapter II.
The street development standard. of most cities provide curb radii of
5 to 30 ft, most of which are between 10 and 15 ft. Most passenger cars
operating at very low speed on Janes 10 ft or more in width are able to
make a right turn with a curb radius of about 15 ft with little
encroachment on other lanes. However, operation of these vehicles at
increased speeds or of larger vehicles even at very low speed generally
results in substantial encroachment on adjacent lanes at either the
beginning or the end of the turn, or both. Where there are curb
parking lanes on both of the intersecting streets and parking is
restricted thereon or for some distance from the corner, the extra
width provided by the restriction serves to increase the usable radius.
On most streets, curb radii of 10 to 15 ft are reasonable because streets
and sidewalks are geJ!erally confined within the public right-of-way,
and larger radii can be obtained only by narrowing sidewalks at corners
and increasing the length of pedestrian crosswalks. However, to ensure
efficient traffic operation on arterial streets carrying heavy traffic
volumes, it is desirable to provide corner radii of 15 to 25 ft for
passenger vehicles and of 30 to 50 ft for most trucks and buses to
expedite turns to and from the through lanes. Where large truck
combinations turn frequently, somewhat larger radii for turns at inter-
sections are required.
Effect of Curb Radii on Turning Paths
The effect of curb radii on the paths of various design vehicles
turning through an angle of 90° on streets without parking lanes is
shown in Figures IX-7 and IX-8. Figure IX-7A shows the effects of a
15-ft radius. With 12-ft lanes, a design passenger vehicle can turn with
no encroachment on an adjacent lane at the exit, but the SU, BUS, and .
WB-40 design vehicles must swing wide on both streets and occupy
two lanes on each street. To turn within two lanes of the cross street,
the WB-50 design vehlcle occupies a part of the third lane on the
approach pavement. The path of a WB-60 design vehicle is not shown
in this figure or the figures that follow because it has a narrower
turning path than a WB-50 design vehicle.
Figure IX-7B shows operation around a 25-ft curb radius. The
passenger vehicle can tum within a 12-ft lane, and all trucks can turn
(by swinging wide) within the confines of two lanes on each street.
Ar-G1
i y e n r e II A t-Gradc ln:t'1 '· · •ins p LSIGJ VEHICLE { 0 Edge of 4-lone slreel I "w0:5<nxsiGl-i-vt"Hfr:ct-: WB-40 ------------------~ __ ..,._,,., __ ,,, __ ,..,,___,,,/ '----.:::S~-~--""--1'1-r--------· --------------· -~ ciOiS-Street' ~ p ,{I i: : : ___________________ __,,,! l : ~ i) -------------------------/ . -_______________________ , I ~-?b-h\ Ni { Cress Sireet' -· .-/~ --~ .l ~ ..; -b-Figure IX-7. Effect of curb radii on turning paths of various design vehicles {diagrams A and Bl. 743
744 AASHTO-Geometric Dc:-;ign <?(Highways and Streets
Figure IX-8C shows op ration around a JO-ft radius. The SU design
vehicle can approach the turn within the right lane and turn into the
second lane on the cross street. A WB-40 design vehicle encroaches
sl ightly on the third lane. The BUS and WB-50 design vehicles must
occupy a portion of the third lane at the exit.
Figure IX-8D shows that all trucks, including the WB-50 design
vehicle, can turn around a 40-ft radius from the outer lane without
encroaching on the third lane of the cross street. This type of maneuver
is practicable for turns from an arterial street where the cross street is
four lanes or wider, because the second lane on the cross street
normally would be free of traffic as a result of signal or stop sign
control on the cross street. Turns from the cross street to the arterial
about such radii also would not be disturbing where signal control is
used, but without such control, drivers of trucks must wait for a proper
gap in traffic to turn into the second lane or. the major street.
Table IX-3 shows the effect of the angle of intersection on turning
paths of various design vehicles on streets without parking lanes. The
dimensions d1 and d2 are the widths occupied by the turning vehicle
on the main street and on the cross street, respectively, while nego-
tiating turns through various angles. Both dimensions are measured
from the right-hand curb to the point of maximum overhang. These
widths, shown for various angles of turn, curb radii, and for two types
of maneuvers, generally increase with the angle of turn. Table IX-3
shows that when a simple, single-radius curve is used for the curb
return, a very large radius must be used or the streets must be very
wide to accommodate the longer vehicles, particularly where the cen-
tral angle is greater than 90°. For this reason three-centered curves (or
offset, simple curves in combination with tapers or spirals to fit the
paths of vehicles properly) are much preferred. Tables IX-1 and IX-2
include curve radii suitable for accommodating the several classes of
design vehicles for a wide range of angles of turn. Data are shown for
simple curves and for two types of three-centered compound curves.
The radii Tor simple curves have been omitted for angles of turn
greater than 90° for the reasons given, although they may be used for
separate turning roadways where sufficient rights-of-way are available
and where there is little pedestrian traffic.
As shown by the footnote for Table IX-4, straight tapered wedges
having a taper of about 15:1 may be substituted for the longer radius
portions of the three-centered curves. A spiral-curve-spiral arrange·
ment may also be used.
At·
"'
1gn
the
h \
sign
11out
llVCt
et is
treet
sign
erial
ol is
oper
11ing
The
hid
1ego-
,ur'd
'hes .
:ype ·
IX-3
curb
very
cen-
~s (or
.t the
IX-2
;es of
;n for
1rves.
·tum
~d for
~ilable
edges
·adius
ange-
Infersections
SU
WB-&l
Poth• of inner whH I
-c-
No porkino lone•
on eilher street
Anc~le of interaection equal to 90 deorees
-----+-1---.. ~-~~-:l Cros~ Str~~t :~50SU40R.~ z;, --· ·-----------
ff/ R•40'
;; I ~ l -d-
(f) : ~ : I -tt It 0 :i ..;
Figure IX-8. Effect of curb radii on turning paths of
various design vehicles (diagrams C and 0).
745
746 AASHTU --Ceon •· '" 1/ ii igliwc1.v:, uud Stru 15: Ar-('•
d2 (ft) for Cetea A snd B Whers: W iti
Design R =16' R=20' R=2t' R=30' R=40' encru.
smali.
Vehicle A B A B A B A B A B :ibkl•
300 SU 14 13 14 13 13 13 13 13 13 13 a<lj<k•
BUS 22 17 19 17 19 17 19 17 18 17 arc l?
WB-40 14 14 14 14 14 14 14 14 14 14 at lea~
WB-50 20 17 20 17 20 17 19 16 18 16 the P1
unless
6()0 SU 19 16 19 16 17 15 16 15 14 14
BUS 28 21 26 20 24 20 23 19 22 18 par kin
WB-40 24 19 22 19 21 19 19 18 17 16 Cautio
WB-50 31 22 27 21 28 29 25 19 22 18 parki n
to the
900 SU 26 20 23 18 19 16 17 15 13 13 hours
BUS 38 23 33 22 30 22 25 21 21 18
WB-40 31 22 27 21 23 21 19 18 17 16 pre\·a;
WB-50 42 22 37 24 34 22 29 21 22 18 For
balan(
120° SU 34 22 27 19 2i 18 17 16 13 13 ing a•
BUS 46 28 40 25 32 23 26 19 19 18 radiu~
WB-40 37 23 29 22 24 22 19 18 17 16
WB-50 50 29 43 28 36 27 30 26 22 18 with\
Cro:
150° SU 40 25 32 21 22 19 17 16 12 12 right-(
BUS 48 28 40 25 32 23 22 18 17 16 On th
WB-40 39 24 29 22 23 22 19 18 17 16 right-~
WB-50 53 31 46 28 36 27 28 26 22 18 the sa
NOTES: P design vehicle turns within 12 ft width where R 15 ft or dist an
more. No parking lanes on either street. street
The
curb I
_...--------,-:TH ~ f-T /'::::===·-,-~ /·:.-~----... ::.~ The d
inters
/ ,,/' 1't ~ ..... :::.:--...... .............. The i / ~--i I l't NTM 0# .... ~ ' ! R ~~; 'ti' why c : ! ..... fllUR ~
: : WHE£L i i ("~ larger
' . \ i CASE 8 .. CASE . ' A i \ desir· ~ VEHICLE TUffNS FROM \ \ TURNING VEHICLE
PROPER LANE ANO SWINGS SWINGS EQUALLY WIDE distan
W10E OH CROSS STREET ON BOTH STREETS ln (
d, •12'; d, is _lab .. d, d,•d11bolh -loble satisf:
practi
the a
Table IX-3. Cross-street width occupied by turning vehicle nu ml--
for various angles of intersection and curb radii. n:cu1.
·'
R=40' -L
13 13
18 17
14 14
18 16
14 14
22 18
17 16
22 18
13 13
21 18
17 16
22 18
13 13
19 18
17 16
22 18
12 12
17 16
17 16
22 18
15 ft or
rlai>lt
1ehicle
ii.
"·
A t-Gr.icle Intersections 747
With curb parking on the streets, vehicles are able to turn without
encroachment on adjacent Janes, even where cu rb ra dii are relatively
small. As shown in Figure IX-9 , the SU and WB-40 design vehicles are
able to turn about a 15-ft curb radius with little if any encroachment on
adjacent lanes where parking lanes are 8 to 10 ft wide and traffic lanes
are 12 ft wide. Parking must be restricted, however, for a distance of
at least 15 ft in advance of the PC on the approach and 30 ft beyond
the PT on the exit. The BUS and WB-50 design vehicles will encroach
unless the curb radius is at least 25 ft, as in Figure IX-9B, and the
parking on the far end of the turn is restricted for 40 ft beyond the PT.
Caution in the use of radii of 15 or 25 ft is advised at first even where
parking can be permitted, because traffic volumes likely will increase
to the point where all parking will be prohibited either during rush
hours or throughout the day, and the same turning conditions will
prevail as shown in Figures IX-7 and IX-8 and Table IX-3.
For arterial street design, adequate radii for vehicles must be
balanced against the needs of pedestrians and the difficulty of acquir-
in g additional right-of-way or corner setbacks. Because the corner
radius often is a compromise, its effect on pedestrians in combination
with vehicular movements should be examined.
Crosswalk distances and the correspondingly required additional
right-of-way or corner setback increase with the radius of curb return.
On the basis of the assumptions that the sidewalk centerline at a
right-angle intersection is in line with the middle of a border and that
the same curb radius is used on all four corners, the added crosswalk
distances between curbs as compared with the normal curb-to-curb
street widths are shown in Figure IX-10.
The additional right-of-way or corner setback resulting from various
curb radii for border widths of 10 and 20 ft is shown in Figure IX-11.
The dimensions shown in Figures IX-10 and IX-11 vary somewhat with
intersection angles that are either more or less than 90°.
The dimensions presented in Figures IX-10 and IX-11 demonstrate
why curb radii of only 10 to 15 ft have been used in most cities. Where
larger radii are used, an intermediate refuge or median island is
desirable or crosswalks may need to be offset so that crosswalk
distances are not objectionable.
In conclusion, corner radii at intersections on arterial streets should
satisfy the requirements of the drivers using them to the extent
practical and in consideration of the amount of right-of-way available,
the angle of the intersection, numbers of pedestrians, width and
number of lanes on the intersecting streets, and amounts of speed
reductions. The following summary is offered as a guide:
I-w w a: I-<~'
a: c -,
<t :;,;
~
12'
12'
., c: 0 _,
"' " ~
0 0..
8'-10
., c: 0 _,
O' .!:'
"' 0 0..
e'-1d
PovelT'enl ( dge
-a-
• Porkmq Restric11on f0r SU
:t Porkmq Res1rict1on for WB-40,
WB-50 and BUS.
Figure IX-9. Effect of curb radii and parking on
turning paths.
, .:j '> r A 1-Grade Intersections
lid
2
Cu rb
I I W/2 W !
\ _rrf= _ ~:,....,S=1d=e=wo=l"--k -~ _ -f-
Curb Radius, R
Feet
10
20
30
40
50
lid ' Increased Walking Distance
Between Curbs Resulting From
Curved Curb Return At Inter-
sections
w =Width Of Border Or The Normal
Setback On The Approach To
An Intersection
R' Radius Of Curb Return
Added C rosswolk 01 stance C.d
W= 10'
Tee1
3
14
2.7
42
57
W=2Q'
Feel
0
5
15
27
40
Figure IX-10. Variations in length of crosswalk with
corner-curb radius and width of border.
749
7Stl
w
-
;~>
r~ ~
,/ .
Aw•Add1tiono1 C'-'<n~t ~t:tCock c.r•r.g
45• rod1c1.
w • '1<'1h of ~O,.dtr .... f Hie r.Nrr.'il
:;etboc"' nr 1ne )C1pr')o-:.r. '-:> or.
t01f'~flC1dt":
I'.. • •
A101t1f"'in.J~rner Setbock,Aw
Curb Rod1us, R w,10· W•20 '
feel fe~' feet
10 0 0
20 4 ()
30 p 4
~o I ~
50 17 13
Figure IX-11. Variations in corner setback with corner-curb
radius and width of border.
1. Radii of
radii ma:
oc't:asion
are park
retain th
parking
crossing
2. Radii of
on new c
3. Radii of
where fr
much en
.:\. Radii of
curves o
ate desi
combina
desirablt
S. Radii di1
or speci;,
-aged, ar
urb radii a
t irning moverr
10 four or mori
by left-turning
MIN
Where the i
· re designed 1
U<. sign permih
the pavement :
proper control
provided to for
traffic turning
At-grade in1
large corner r
encourage ha.
pedestrian ere
'-!mp le interse
L 1 ': can wand
i 1 Juu:·d in ext
mer-curb
At-Grade Intersections 751
1. Radii of 15 to 25 ft are adequate for passenger vehicles. These
radii may be provided at minor cross streets where there is little
occasion for trucks to turn or at major intersections where there
are parking lanes. Where the street has sufficient capacity to
retain the curb lane as a parking lane for the foreseeable future,
parking should be restricted for appropriate distances from the
crossing.
2. Radii of 25 ft or more at minor cross streets should be provided
on new construction and on reconstruction where space permits.
3. Radii of 30 ft or more at major cross streets should be provided
where feasible so that an occasional truck can turn without too
much encroachment.
4. Radii of 40 ft or more, and preferably three-centered compound
curves or simple curves with tapers to fit the paths of appropri-
ate design vehicles, should be provided where large truck
combinations and buses turn frequently. Larger radii are also
desirable where speed reductions would cause problems.
5. Radii dimensions should be coordinated with crosswalk distances
or special designs to make crosswalks safer for pedestrians, the
aged, and the handicapped. (See Chapter IV.)
Curb radii at corners on two-way streets have little effect on left-
turning movements. Where the width of an arterial street is equivalent
to four or m?re lanes, generally there is no problem of encroachment
by left-turning 'Vehicles.
MINIMUM DESIGN FOR TURNING ROADWAYS
Where the inner edges of pavement for right tut:ns at intersections
are designed to accommodate se~itrailer combinations -0r where the
design permits passenger vehicle$ tQ tum at speeds of 15 mp}) or more,
the pavement area at the intersection may become excessively large for
proper control of traffic. To avoid this condition an island should be
provided to form a separate turning-roadway (a connecting roadway for
traffic turning between two intersection legs).
At-grade intersections. have large paved areas, such as those with
large corner radii and those at oblique angle crossings, permit and
encourage hazardous, ·uncontrolled vehicle movements, require long
pedestrian crossings, and have unused pavement areas. Even at a
si mple intersection, appreciable areas may exist on which some vehi-
cles can wander from natural and expected paths. Conflicts may be
reduced in extent and intensity by layout design to include islands.
758 AASHTO-Geometric Design of Highways and Streets
and width of crosswalks, the location and size of transit loading zones
1 ' and the provision of wheelchair ramps influence the size and location of. · :
the refuge islands. Chapter IV contains details of wheelchair ramp , ··2 ,;~
design that affect the minimum size of the small islands. In rural as· " :(,
well as in urban areas many of the islands designed for channelization
are of the type and location to serve as refuge for pedestrians. Islands
a, b, c, e, and fin Figure IX-12 are examples. The general principles
for island design apply directly to refuge islands, except that barrier
curbs usually are considered necessary.
Island Size and Designation
Island sizes and shapes vary materially from one intersection to
another, as shown in Figure IX-12. Further variations, not illustrated,
occur at multiple and acute-angle intersections. Islands should be
sufficiently large to command attention. The smallest curbed island
that normally should be considered is one that has an area of approxi-
mately 50 ft2 for urban streets, and 75 ft2 for rural intersections.
However, 100 ft2 is preferable for both. Accordingly, triangular islands
should not be less than about 12 ft, and preferably 15 ft, on a side after
the rounding of corners. Elongated or divisional islands should be not
less than 4 ft wide and 20 to 25 ft long. In special cases where space is ..
limited, elongated islands, as in b and g of Figure IX-12, may be ;, .
reduced to an absolute minimum width of 2 ft. In general, introducing
curbed divisional islands at isolated intersections on high-speed higb-t
ways is undesirable unless special attention is directed to providing:
high visibility for the islands. Curbed divisional islands introduced 1;t
isolated intersections on high-speed highways should be at least 100.ft
and preferably several hundred fe et in length. When they cannot be '.
long, they may be preceded by visibly roughened pavement, jiggle,
bars, or marking. When situated in the vicinity of a high point in the :-
roadway profile or at or near the beginning of a horizontal curve, th«r ·-
approach end of the curbed island should be extended to be clearly,~
visible to approaching drivers. ··
Islands should be delineated or outlined by a variety of treatments, ·
depending on their size, location, and function. The type of area in ·;
which the intersection is located, rural versus urban, also governs the ·
design. In a physical sense, islands can be divided into three groups: J. ...
(1) raised islands outlined by curbs, (2) islands delineated by pavement
markings, buttons, or raised (jiggle) bars placed on all-paved areas,'\.
and (3) nonpaved areas form ed by the pavement edges-possibly :
\ .!
At-1
sup
mot
c
pos
ofte
urb
I!
wht
use
und
haz
wht
isl a
1
and
Jar!
1
veg
det
obs
Vln•
con
but
dei:
esp
SnG
con
isl a
sirr
Sec
Lai
tex
po!
sh<
wh
pe1
IV.
'eets
>nes,
on of
ramp
al as
ation
lands
:iples
irrier
on to
rated,
Id be.
island
proxi·
tions.
;lands
: after
>e not
1ace is
ay be
lucing
~t
high_:-.,,. . 'd' , . \ .. ,, ~dmat .:" ,,·~· ~:· ce a · · ·
100 ft
not be
jiggle
in the
re, the_ -
clel9"1Y, :·~·
";-·~
ments,-
lrea in
ms the
areas,
ossibly
At-Grade Intersections 759
supplemented by delineators on posts or other guideposts-or a
mounded-earth treatment beyond and adjacent to the pavement edges.
Curbed island treatment, group l, is universal and is the most
positive. In rural areas where curbs are not common, this treatment
often is limited to islands of small to intermediate size. Conversely, in
urban areas, use of this type of island is generally standard practice.
Island delineation on paved areas, group 2, is used in urban districts
where speeds are low and space is limited. In rural areas this type is
used where maintenance problems and snow removal make curbs
undesirable or where high approach speeds make any curb a potential
hazard. Group 2 islands also are applicable on low-volume highways
where the added expense of curbs may not be warranted and where the
islands are not large enough for delineation by pavement edges alone.
The group 3 treatment necessarily applies to other than small islands
and is logical primarily at rural intersections where there is space for
large-radius intersection curves.
The central areas of large islands in most cases have a turf or other
vegetative cover. As space and the overall character of the highway
determine, low landscape planting may be included, but it must not
obstruct sight distance. Ground cover or plant growth, such as turf,
vines, and shrubs, is desirable for an island and provides excellent
contrast with the paved areas. Small curbed islands may be mounded,
but where pavement cross slopes are outward, large islands should be
depressed to avoid draining across the pavement. This feataure is
especially desirable where alternate freezing and thawing of stored
snow may occur. For small curbed islands and in areas where growing
conditions are not favorable, some form of paved surface is used on the
island. In many respects the curbed-island cross section design is
similar to that discussed in the section "Medians-Width and Cross
Section'' in Chapter IV.
Delineation and Approach-End Treatment
Delineation of small curbed islands is effected primarily by curbs.
Large curbed · islands may be sufficiently delineated by color and
texture contrast of vegetative cover, mounded earth, shrubs, guard
posts, signs, or any combination of these. In rural areas island curbs
should nearly always be a mountable type, as in Figure IV-3, except
where there is a definite need for a barrier, as at structures or
pedestrian crossings. In special cases, barrier curbs, as in Figures
IV-3A and IV-3C. are suitable, preferably of heights not more than 6 to
760 AASHTO-Geometric Design of Highways and Streets ·
7 in. Either type of curb could be appropriate in urban areas, depend-
ing on the condition. High-visibility curbs are advantageous at hazard-
ous locations or on islands and roadway forks approached by high.
speed traffic. , :
The outline of a curbed island is determined by the edge of through-.
traffic lanes and turning roadways, with lateral clearance, if any, to the
curbed island sides. The points at the intersections of the curbed island
are rounded or beveled for visibility and construction simplicity. The
amount that a curbed island is offset from the through-traffic lane is
influenced by the type of edge treatment and other factors such as
island contrast, length of taper or auxiliary pavement preceding the
curbed island, and traffic speed. Island curbs are introduced rather
suddenly and should be offset from the edge of through-traffic lanes
even if they are mountable. A mountable curb at an island need not be-
offset from the edge of a turning roadway, except to reduce its :.""'-
vulnerability. Barrier curbs should be offset from edges of through and ·
turning roadway pavements.
Details of triangular curbed island design are shown in Figures IX-14
and IX-15 . The lower right corner of each curbed island is designed for
approach-end treatment. Three curbed island sizes, small, intermedi·
ate, and large, are shown for two general cases of through-traffic lane
edge: (1) The curbed island edge is determined by offset clearance
from the through pavement, and (2) the curbed island edge is at the
outside of a shoulder carried through the' intersection. Small curbed
islands are those of minimum or near-minimum size, as previously
discussed. Large curbed islands are those with side dimensions at least. ·
100 ft long. All curbed islands in Figures IX-14 and IX-15 are sho_wn~ ;: ·~ ., with approach noses rounded on appropriate radii of 2 to 3 ft and witM/~
minimum rounding (about 1-ft radius) at merging ends. The lower left, ..
corner is rounded with a radius of 2 to 5 ft.
Figure IX-14 shows curbed islands adjacent to through-traffic lanes.
Where there are no curbs on the approach pavement, the minimum_ c
offset of the edge of the curbed island should be 2 to 3 ft. Where the _
approach pavement has a mountable curb, a similar curb on the curbed~.-·~.
island could be located at the edge of the through lane where there is '·
sufficient length of curbed island to effect a gradual taper from ·the'~:
nose offset. Barrier curbs should be offset from the through pavement .\
edge, regardless of the size of the curbed island, to avoid a sense-·of :.
lateral restriction to drivers. Where the intermediate and large-size(!··.·
islands are uncurbed, the indicated offsets of the curbed island proper .-
are desirable but not essential.
The approach end of a curbed island should be conspicuous to·
approaching drivers and should be definitely clear of vehicle paths,
At-Grade ln
Fl
r
e
e
e
d
y
;t
•• m
lC
:d
is
ie
nt
of
ed
er
to
IS,
At-Grade /niersections
:i: I I
::~-1· I>
I I 2·~ R=Z'
2'~=========
Nole ' Layouts Shown Also Apply To Large And
Intermediate Islands Without Curbs, Island
Side Of fsels Desirable But May Be Omitted.
C=:l Painted Stripes, Contrasting Surface,
Jiggle Bars, Etc.
Z'To 3'-Alt. With Mountable
Curb (See Text) Offset
CVRBED ISLANDS -NO SHOULDER
Figure IX-14. Details of triangular island design
(curbed islands, no shoulder).
761
762 AASHTO-Geometric Design of Highways and Streets
Nole ' Layouts Shown Also Apply To Lorge And
Intermediate Islands Without Curbs, Island
Side Offsets Desirable But Moy Be Omi1ted.
~Shoulder
R= i'To 1.5°
INTERMEDIATE
'.b._ 2' To 3' Offset
-==--~Th;~~gh Traffic
Lone
R= l0To 1.5'
CURBED ISLANDS WITH SHOULDERS
Figure IX-16. Details of triangular Island design
(curbed Island with shoulders).
A t-Grade It.
physically a
raised deli1
particularly
by vehicles
be greater
ft. For me<
nose shoul1
median ed1
gradually'
nose offse
pavement :
Large offst
by a decel1
When a
offset from
width, as
high and 1
gradual wi
nose of I;
developed
barrier cu
tapered, ~
Curbed
warning t•
ings, roug
are partic1
IX-14. To
be used,
ends of c1
tors mour
located in
the MUT1
Deline~
ends of :
approach·
gradually
frequent\
areas als•
of color a
bars tha1
section s
IX-16 de
?ts At-Grade Inters ections 763
physically and visually, so that drivers will not veer from the island. A
raised delineator (nonrigid) may be desirable on the approach end,
particularly if the curbed island is so narrow that it could be straddled
by vehicles. The offset from the travel lane to the approach nose should
be greater than that to the side of the curbed island, normally about 2
ft. For median curbed islands the face of curb at the approach island
nose should be offset at least 2 ft and preferably 4 ft from the normal
median edge of pavement. The median curbed island should then be
gradually widened to its full width. For other curbed islands the total
nose offset should be 4 to 6 ft from the normal edge of through
pavement and 2 to 3 ft from the pavement edge of a turning roadway.
Large offsets should be provided where the curbed island is preceded
by a deceleration lane or a gradually widening auxiliary pavement.
When an approach shoulder is used, the curbed island should be
offset from the through travel lane by an amount equal to the shoulder
width, as shown in Figure IX-15. Where speeds are intermediate or
high and the curbed island is preceded by a deceleration lane or a
gradual widen ing auxiliary pavement, it may be desirable to offset the
nose of large curbed islands an additional 2 to 4 ft. In heavily
developed areas an offset as small as 1 ft may be appropriate for
barrier curbs where the approach nose of the curb island is offset and
tapered, particularly where pedestrian traffic is a factor.
Curbed islands should be provided with devices .to give advance
warning to approaching drivers, both day and night. Pavement mark-
ings, roughened pavement, or jiggle bars in front of the approach nose
are particularly advantageous on the areas shown as stippled in Figure
IX-14. To the extent practicable, other high-visibility indications should
be used, such as reflectorized curbs, signs located near the approach
ends of curbed islands suitably reflectorized or illuminated, or reflec-
tors mounted above the curbed island surface. The curbs of all islands
located in the line of traffic flow should be marked in accordance with
the MUTCD (2).
Delineation and warning devices are especially pertinent at approach
ends of median-curbed islands, which usually are in direct line of
approaching traffic. In rural areas the approach should consist of a
gradually widening center stripe (Figure IX-16). Although not as
frequently obtainable, this approach should be strived for in urban
areas also. Preferably, it should gradually change to a raised marking
of color and texture contrasting with that of the traffic lanes or to jiggle
bars that may be crossed readily even at considerable speed. This
section should be as long as practicable. The cross sections in Figure
IX-16 demonstrate the transition. The face of curb at the approach
764 AASHTO-Geometn"c Design of Highways and Streets
island nose should be offset at least 2 ft and preferably 4 ft from the
normal (median) edge of pavement, and the widened pavement
gradually should be transitioned to the normal width toward the
crossroad. The end of the curbed island at the crossroad is designed as
a median end. The delineation and approach-end treatment described
for triangular and median-curbed islands generally apply to other.
forms of curbed islands. However, pavement markings or raised pave-
ment areas at approach island ends may not be necessary at secondary
curbed islands situated within a multiple-island intersection. The ap-, ..
proach-end treatment is primarily· pertinent to curbed islands first <
approached by traffic entering an intersection.
Right-Angle Tums With Comer Islands
The principal controls for the design of turning roadways are the
alinement of the inner pavement edge and the pavement width, so that
the design vehicle can be accommodated while turning at low speed.
With radii greater than minimum, these controls res ult in an area large
enough for an island, generally triangular in shape, between the inner
edge of the turning roadway and the pavement edges (extended) of the
two through highways. Such an island is desirable for delineating the
path of through and turning traffic, for the placement of signs, and for
providing refuge for pedestrians.,Larger islands may be necessary to
locate signs and to facilitate snow-removal operations. The inner edge
of pavement on the turning roadway should be designed to provide at
least the minimum island and the minimum width of turning roadway
pavement. The turning roadway pavement should be wide enough to
permit the outer and the inner wheel tracks of a selected vehicle to be
within the edges of the pavement by about 2 ft on each side. Generally,
the turning roadway pavement width should not be less than 14 ft.
Figure IX-17 shows minimum designs for turning roadways for a 90°
right tum to fit these controls. A design based on a minimum island
and a minimum width of channel of 14 ft (Figure IX-17 A) results in ·a
circular arc of 60-ft radius (not shown) en the inner pavement edge of
the turning roadway or in a three-centered curve (as shown) with radii
of 150, SO, and 150 ft with the middle curve offset 3 ft from the tangent
edges extended. This design not only permits passenger vehicles to
tum at a speed of 15 mph but also enables SU design vehicles to turn
on a radius (outer front wheel) of approximately 65 ft and still clear the
pavement edges of the channel by about 1 ft on each side, as shown by
the vehicle paths in the figure.
Ac-G
I Streets
i'om the
avement
rard the .
igned as
.escribed
to other
ed pave-
econdary -,'
The ap~ .,
nds first
; are the
1, so that
w speed.
1rea large
the inner
~d) of the
:ating the
s, and for
:essary to
mer edge
>rovide at
: roadway
~nough to
1icle to be
:Jenerally.
14 ft.
s for a 90°
um island
~suits in a
nt e_dge of
with radii
tie tangent
rehicles to
!es to turn
11 clear the
; shown by
At-Grade Intersections
Raised Median Island Raised Transition Approach To
With Curbs Island ; Color And Texture Con-
Normal Pavement
PR.C "1 frosting With Normal Pavement . PC
~
J
PLAN
Highway~
Median Island Area Probably
Grossed.
SlopinQ Curb Except At
Pedestrian C~ossing.
SECTION AT PT.
Median Island Area Probobl
Grossed .
Raised Transition Approach To
Island; Color And Texture Con-
trast in With Normal Pavement .
Curb
SECTION BETWEEN PT AND PRC.
Raised Transition Approach To I Island; Color And Texture Con-
~;~~11~;:;.>
1
tra:;z1~::~0;ormol Pavement
SECTION AT PRC.
DIAGRAMMATIC CROSS SECTIONS
Figure IX-16. Details of divisional island design.
765
766 AASHTO-Geometric Design of Highways and Streets
3-CENTERED CURVE•
150°-50°-150°,0FFSET 3°
~-EQUIVALENT SIMPLE C\1RVE RADIUS 60°
-A-
WB-50 SEMITR. COMB. PATH
OUTER RADIUS 75· t
SINGLE UNIT TRUCK PATH
OUTER RADIUS 70'!
3-CENTERED CURVE•
150' -so' -150',0FFSET 5'
EQUIVALENT SIMPLE CURVE RADIUS 70°
-B-
WB-50 SEMITR. COMB.
3-CENTERED CURVE • 1ao'-65°-1ao', OFFSET 6°
EQUIVALENT SIMPLE CURVE RADIUS 100° i
-c-
/
Figure IX-17. Designs for turning roadways with
minimum corner Island.
., ......
At-Gru
By i
combir
tangen
IX-l 7B
turnini
WB-5(
adj ace
At l<
partict
Figure:
curve
gen er:
turn in
er veh
The
paven
highw
When
curbs.
prior
be m<
allow
Figur·
For
symrr
comp·
pro vi•
of a!
latter
shoul
M
a ba
Curv
men
chos
part
mu11
of tI
ets At-Grade Intersections 767
By increasing the pavement width to 18 ft and using the same
combination of curves but with the middle curve offset 5 ft from the
tangent edges extended, a more desirable arrangement results (Figure
LX-178). This design enables the SU design vehicle to use a 70-ft
turning radius with liberal clearances and makes it possible for the
WB-50 vehicle to negotiate the tum with only slight encroachment on
adjacent through-traffic lanes.
At locations where a significant number of semitrailer combinations,
particularly the longer units, will be turning, the arrangement shown in
Figure IX-17C should be used. This design, consisting of a minimum
curve of 65-ft radius, offset 6 ft, and terminal curves of 180-ft radii,
generally provides for a WB-50 design vehicle passing through a 20-ft
turning roadway pavement and greatly benefits the operation of small-
er vehicles.
The island in all instances should be located about 2 ft outside the
pavement edges extended, as shown in Figure IX-17C. For high-speed
highways the offset to the through Janes would be desirably greater.
When of minimum or near-minimum size, it should be delineated by
curbs. In rural areas the use of the painted islands should be given
prior consideration. If curbed islands are necessary, the curbs should
be mountable to make them less hazardous to through traffic and to
allow for greater latitude in the maneuvering of large vehicles. (See
Figures IX-14 and IX-15 and accompanying discussion.)
For each minimum design shown in Figure IX-17. a three-centered
symmetric compound curve is recommended; however, asymmetric
compound curves could also be used, particularly where the design
provides for the turning of trucks. Although an equivalent simple curve
of a given radius is noted in the figure in each case, its use in the two
latter designs may result in design vehicle encroachments on the
shoulder or the island.
Oblique-Angle Tums With Comer Islands
Minimum design dimensions for oblique-angle turns, determined on
a basis similar to that for right-angle turns, are given in Table IX-4.
Curve design for the inner edge of pavement, turning roadway pave-
ment width, and the approximate island size are indicated for the three
chosen design classifications described at the bottom of the table. For a
particular intersection the designer may choose from the three mini-
mu m designs shown in accordance with the size of vehicles, the volume
of traffic anticipated, and the physical controls at the site.
768 AASHTO-Geometric Design of Highways and Streets
Three-Centered Width Approx'. Compound Curve Angle of Island
of Turn Design Radii Offset Lane Size
(degrees) Classification (ft) (ft) (ft) (sq ft)
75 A 150-75-150 3.5 14 60 ..
B 150-75-150 5.0 18 50 c 180-90-180 3.5 20 50
A 150-50-150 3.0 14 50
B 150-50-150 5.0 18 80 c 180-65-180 6.0 20 125
105 A 120-40-120 2.0 15 10
B 100-35-100 5.0 22 50 c 180-45-180 8.0 30 60
120 A 100-30-100 2.5 16 120
B 100-30-100 5.0 24 90 c 180-40-180 8.5 34 220
135 A 100-30-100 2.5 16 460
B 100-30-100 5.0 26 370
c 160-35-160 9.0 35 640
150 A 100-30-100 2.5 16 1400
B 100-30-100 6.0 30 1170
c 160-35-160 7.1 38 1720
a111ustrated in Figure IX-17.
NOTES: Asymmetric three-centered compound curves and straight
tapers with a simple curve can also be used without signifi-
cantly alte·ing the width of pavement or corner island size.
Painted island delineation is recommended for islands less
than 75 ft2 in size.
Design classification:
A -Primarily passenger vehicles; permits occasional
design single-unit truck to turn with restricted clear-
ances.
B -Provides adequately for SU; permits occasional
WB-50 to turn with slight encroachment on adjacent
traffic lanes.
C -Provides fully for WB-50.
Table IX-4. Minimum designs for turning roadways.
60 I
120
90
220
460
370
640
1400
1170
1720
traight
signifi·
:e.
jg less
asional
l clear·
:asional
djacent
A r-Grade Intersections 769
In Table IX-4 no design values are given for angles of turn less than
75 °. Turning roadways for flat-angle turns involve relatively large radii
and are not considered in the minimum class. Such arrangements
require individual design to fit site controls and traffic conditions. For
angles of turn between 75° and 120° the designs are governed by a
minimum island, which provides for turns on more than minimum-
turning radii. For angles of 120° or more the sharpest turning paths of
the selected vehicles and arrangements of curves on the inner edge of
pavement to fit these paths generally control the design, ·the resulting
island size being greater than the minimum. The inner edge of
pavement arrangements for designs Band C for turning angles of 120°
to 150° are the same as those given in Table IX-2 for single-unit trucks
and 50-ft semitrailer combinations, respectively. The size of islands for
the large turning angles given in the last column of Table IX-4
indicates the otherwise unused and uncontrolled areas of pavement
that are eliminated by the use of islands.
APPLICATION AT TURNING
ROADWAY TERMINALS
An important part of intersection design is the provision of a suitable
pavement edge alinement where a turning road~ay departs from or
joins the through-highway pavement. Ease and smoothness of opera-
tion result when the pavement edge is designed with spirals or
compound curves and is of a shape and length to avoid abrupt
deceleration by drivers before they leave the through pavement, to
permit development of superelevation in advance of the maximum
curvature, and to enable ·vehicles to follow natural turning paths.
Various degrees of transitional treatments for an exit from a highway
are illustrated in Figures IX-18 and IX-19 for design (turning) speeds of
20 and 30 mph, respectively. As p, the offset from the edge of the
through pavement to the minimum-radius curve produced, is in-
creased, progressively smoother and more adequate turnout facilities
are provided. A spiral curve of minimum length (Table IX~5) joining
the edge of through pavement with the interS'ection curve (Figure
IX-18B) is a substantial improvement over the simple curve (Figure
IX-18A) in easing the turning path. This short spiral is not satisfactory
for development of superelevation on the turnout because the wedge of
auxiliary pavement, areas a-b-c, is too small for this purpose. By
doubling the minimum length of spiral (Figure IX-18C), p is increased
to nearly 9 ft, an increase that provides for a more directional path and
874 AASHTO-Geometric Design of Highways and Streets
AUXILIARY LANES
General Design Considerations
From the foregoing discussions it is appropriate to deal with the
design elements of auxiliary lanes with respect to median openings.
Auxiliary lanes are used preceding the openings as described herein.
However, auxiliary lanes are also applied to at-grade intersections
preceding and following right-turning movements and to other pur-
poses supplementary to through-traffic movements.
Auxiliary lanes should be at least 10 ft wide and may be 12 ft wide.
Where curb and gutter sections are used, the gutter-pan width may be
included as a part of the minimum width of the auxiliary lane. When
the auxiliary lane is to be used as a running lane, the necessary curb
offset should be excluded from the gutter-pan width that may be
applied to achieve the required auxiliary-lane width. Desirably, the
Jane width should be in addition to that of the gutter pan. The length of
the auxiliary lanes for turning vehicles consists of three components:
1) deceleration length, (2) storage length, and (3) entering taper.
Desirably, the total length of the auxiliary lane should be the sum of
the length for these three components. Common practice, however, is
to accept a moderate amount of d~celeration within the through lanes
and to consider the taper as a part of the deceleration length_ Where
intersections occur as frequently as four per mile, it is customary to
forego most of the deceleration length and to provide only the storage,
length plus taper. Each component of the auxiliary length is discussed •..,
in the following section.
Deceleration Length
Provision for deceleration clear of the through-traffic lanes ~s ,a
desirable objective on arterial roads or streets and should be i3icor<
porated in design wherever feasible. The length required is :th~(~
needed for a comfortable stop from a speed that is typical of thC '--;_fe ,
average running speed on the main facility. Factors entering into the ... ~;,·1
computation of deceleration distances are discussed in Chapter X. On :"' ~-
the basis of these factors and average running speeds of 20, 30, 40, · .
and 50 mph deceleration lengths at'e 60, 250, 370, and 500 ~. .
respectively. :··~:~:~
These lengths include the length of taper, which is discussed sepa· 'f· ··j:.;J.
rately. On many urban facilities it will not be feasible to provide the .;. 'c~~ ~
~ ~
At-G.
full I
decel•
Howe
shoul·
show1
usual"
Tht
vehicl
lengtl
stopp
fi cien
vehicl
for a
At
may l
aver a.
ment,
over l
car ar
some
comp!
turn,
turnir
At
the si.
of arr
is a f
usu all
of vel
desig1
occur
provi~
Tra
(2).
Wh
age It
single
d Streets
with the
)penings.
d herein.
!rsections
1ther pur-
2 ft wide.
th may be
ne. When
;sary curb
t may be
rably, the
! length of
mponents:
taper.
:he sum of
owever, is
mgh lanes
th. Where
;tomary to
he storage
; discussed
lanes is a
1 be incor·
·ed is that
1ical of the
ng into the
tpter X. On
20, 30, 40,
ITTd 5()() ft,
ussed sepa·
provide the
At-Grade Intersections 875
full length for deceleration. In such cases at least a part of the
deceleration must be accompl ished before enterin g the auxiliary lane.
However. the lengths given should be accepted as a desirable goal and
should be provided where practical and feasible. Deceleration lengths
shown are applicable to both left-and right-turning lanes, but speed is
usually lower in the right lane than in the left lane.
fi,,.. o.. ri~-f1-.r11 lc.-...L
~ ~u·~ cJ.,...,·l"e.. 1 +-<-..i~
Storage Length nor cr11-1·~. Stvr-"-$t
""":..d.. ~ = 2.S' C)r-V>\-e'
The auxiliary lane should be sufficiently lo g store the number of Vl!"iicU-
vehicles likely to accumulate during a critical period. The storage
length should be liberal to avoid the possibility of left-turning vehicles
stopping in the through lanes. The storage length should be suf-
fici ently long so that the entrance to the auxiliary lane is not blocked by
vehicles standing in the through lanes waiting for a signal change or
for a gap in the opposing traffic flow.
At unsignalized intersections the storage length, exclusive of taper,
may be based on the number of turning vehicles likely to arrive in an
average 2-min period within the peak hour. As a minimum require-
ment, space for at least two passenger cars should be provided; with
over 10 percent truck traffic, provision should be made for at least one
car and one truck. The 2-min waiting time is somewhat arbitrary, and
some other interval that depends largely on the opportunities for
completing the left-turn maneuver might be used. These intervals, in
turn, depend on the volume of opposing traffic. Where the volume of
turning traffic is high, a traffic signal will usually be required.
At signalized intersections the required storage length depends on
the signal cycle length, the signal phasing arrangement, and the rate
of arrivals and departures of left-turning vehicles. The storage length
is a function of the probability of occurrence of events and should
usually be based on one and one-half to two times the average number
of vehicles that would store per cycle, which is predicated on the
design volume. This length will be sufficient to serve heavy surges that
occur from time to time. As in the case of unsignalized intersections,
provision should be made for storing at least two vehicles.
. Traffic signal design fundamentals are discussed further in MUTCD
(2).
Where turning lanes are designed for two-lane operation, the stor-
age length is reduced to approximately one-half of that required for
single-lane operation.
: '"'! -~!;
f ·I ''
•I , , . '\. . '
876 AASHTO-Geometric Design of Highways and Streets
Taper
On high-speed highways the taper of an auxiliary lane should
generally conform to that discussed under "Ramp Terminals" in the
section "Speed-Change Lanes" in Chapter X. However, on most urban
streets approaching at-grade intersections, shorter tapers are satisfac-
tory because of lower operating speeds.
Some States permit the tapered section of deceleration auxiliary
lanes to be constructed in a "squared-off" section at full paving width
and depth. This construction requires a painted delineation of the taper
and is only applicable to noncurbed sections. The design requires
transition of the outer or median shoulders around the squared-off
beginning of the decleration Jane.
The squared-off design principle can be applied to median decelera-
tion Janes, and it can also be used at the beginning of deceleration
right-turn exit terminals when there is a single exit lane. When two or
more exit lanes are used, the tapered designs discussed in the section
"Speed-Change Lanes" in Chapter Xis recommended.
Some State Highway agencies believe that the abrupt squared-off
beginning of deceleration exits offers improved driver commitment to
the exit maneuver and also contributes to driver security and safety
because of the elimination of the unused portion of long tapers. For
example, a 15: 1 taper ratio is recommended for operating speeds up to
50 mph. If the auxiliary lane was 12 ft wide, the taper length would be
180 ft. This approach section could be 12 ft wide and constructed for its
full length. The painted delineation taper could be established on a 4:1
ratio that would produce a stripe approximately 50 ft long. This design
principle could be applied to all deceleration design speeds and should
operate with relatively quick exit maneuvers to the full-width lane
under low operating speeds and still provide for the longer exit
maneuvers at the higher operating speeds.
The longitudinal location along the highway, where a vehicle will
move from the through lane to a full-width deceleration lane, will vary
depending on many factors. These factors include the type of vehicle,
the driving characteristics of the vehicle operator, the speed of the
vehicle, weather conditions, and lighting conditions. , .
This design principle could also improve capacity by offering an · ' .. ::;. i::
early exit'to the daily users that would diminish slow-down interference
to the through movements.
The discussions to follow and figure references do not relate to the
"squared-off' design section. However, the design principles de-·· . .,.,#,·.,
scribed in the preceding can be applied.
Ar-Grade ln11
Straight-lir
The taper rat
for operating
end, as sho'
construction.
should be ab<
shows a tape;
or more is n(
and taper (1 )
has the turn
(Figure IX-~
and (2) are ~
configuration
as weJI as tel
A median
change of I<
pavement w'
venience, ar
on divided h
therefore, sl
openings wl
vehicular sp
median are :
Median w
with single 1
adequate a,n
width of at Ir
lanes and a
traveled Jan
buttons or i:
opposing th:
the intersec1
Figure ·IX
within a ffi(
recommend
Figure IX-6<
width of 16
Figure IX-
d Screets
e should
;" in the
ost urban
: satisfac-
auxiliary
ing width
the taper
requires
1uared-off
decelera-
celeration
1en two or
he section
~uared-off
.1itment to
md safety
ipers. For
~eds up to
1 would be
:ted for its
:don a 4:1
his design
md should
Nidth lane
)nger exit
ehicle will
!, will vary
of vehicle,
eed of the
1ffering an
1terference
:late to the
1ciples de-
'·
Ac-Grade Intersections
Straight-line tapers are frequently used, as shown in Figure IX-65A.
The taper rate may be 8: 1 for operating speeds up to 30 mph and 15: 1
for operating speeds of SO mph. A short curve is desirable at either
end, as shown in Figure IX-658, but may be omitted for ease of
construction. Where curves are used at the ends, the tangent section
should be about one-third to one-half of the total length. Figure IX-65C
shows a taper with a symmetrical reverse curve; a length of about 90 ft
or more is necessary. At low speeds, taper (2) will operate as desired
and taper (1) probably will not. A more desirable reverse-curve taper
has the turnoff curve radius about twice that of the second curve
(Figure IX-65D). When 100 ft or more in length is provided, tapers (1)
and (2) are suitable for low-speed operation. All the dimensions and
configurations shown in Figure IX-65 are applicable to right-turn Janes
as well as left-tum lanes.
Median Left-Tum Lanes
A median left-tum Jane is an auxiliary lane for storage or speed
change of left-turning vehicles located at the left of one-directional
pavement within a median or divisional island. Serious hazard, incon-
venience, and considerable loss in efficiency of operation are evident
on divided highways where such lanes are not available. Median lanes,
therefore, should be provided at intersections and at other median
openings where there is a high volume of left turns or where the
vehicular speeds are high. Median lane designs for various widths of
median are shown in Figures IX-66 and IX-67.
Median widths of 20 to 25 ft or more are desirable at intersections
with single median lanes, but widths of 16 to 18 ft permit reasonably
adequate arrangements. Where two median lanes are used, a median
width of at least 28 ft is desirable to permit the installation of two 12-ft
lanes and a 4-ft separator. Although not equal in width to a normal
traveled lane, a 10-ft lane with a 2-ft curbed separator or with traffic
buttons or paint lines, or both, separating the median lane from the
opposing through lane may be acceptable where speeds are low and
the intersection is controlled by traffic signals.
Figure IX-66A shows a minimum design for a median left-turn lane
within a median 14 to 16 ft wide. A curbed divider width of 4 ft is
recommended and the median left-tum lane should be 10 to 12 ft wide.
Figure IX-668 shows a typical median left-turn design within a median
width of 16 to 18 ft. The only change in this design from the design in
Figure IX-66A is a 2-ft minimum offset to the approach nose. Figure
~ ~ ,µ 0 a USEfiiiS 'iiiiii.LfORoOMESTIC S#li'ifffiTsAiiioiSit11ibrrs FIUIM l'llEifOiicifffrfiif'iii.r-' ~--FILL OUT l'llRPLE AREAS. FDR ASSISTAllCE, CALL IDO-Z31·5355 TOLL FREE. " SEE IACK OF FORM SET FOR COMPLETE PREl'AllAnOll lllSTRUCTIOllS. I SENDER'S FEDERAL EXPRESS ACCOUNT NUMBER I I DATE I Aecipienfs Phone Numbef (Very 1mpor111n~ Department/Floor No. State City T~ AIRBILL NO. 1,37a3aoa --· ...._____
,._J YOUR llLLlllS REFEREllCE lllFORMATIOll {FIRST 24 CHARACTERS WILL Al'l'EAR 011111ro1cE.J D ri:: i~~:edEx Acct No,D ~: l~~,:,a~~Ex Acct No. D ~~ ~= ~w State FedEx Acct No. or Major Ctedrt Card No. SERrlCES CHECK OllLY OllE BOX OELlrERY AllD Sl'ECIAL HAllOLlllS CHECK SERrlCES REQUIRED NC/lMES I lfflll#T rou1 OECLAIE• jorER I Z/I' •Zip Codfi of Street Address Requ· JALUE SIZE 0 l'RIORITY 1 O DrERlllSH.T 1 Ovemight 0e1;very 6 LETTER \hlng'four~ fOurP9cklgino)ra12" DrER#lfl#T DEUrERY 0 USl#fl OUR l'ACKABllfti 2 Courie<-Pak Overnight En~ 12''x15W' 30~~~~'3· Ao 4~~"s!.~~· eO STAllOARO AIR so=~=:; _,, ~!!!'f.£~!!.,E!'!__,.,. In moet IOcallona. tt may lake two OJ tnore butlneu dl)'I if Ile =o·~~T,:~cteyornoc ....,INrlMCOl"ldbullMNdly 1trnaylMtlltw•0tmorw~ daysiflhlldellnllllionilOl.lllicMourpnnwy.....,q ..... 1 0 #OLD FOi l'ICK·Ul'G/ve the F-aj Express addrass ;:•you want paci<age held in I r I ( I I· I c:mp. NO. • I U< r rlf*t n "••h "~-·-' 2 ct DELlrER WEEKDAY ~ 0 DEL/rER SATURDAY tEmcna.go-1 Totiif 4 0 ~'!'l!:.n!.~=!:.::i" . G 5 O CllllSTA#T SUllYEILLAllCE SEllYICE /CSS/ I Rec J At' I I City 1 ·:;i • State (El<1ra charge applies) et_V ~ \ 6 0 DRY ICE ---Lbs. 7 0 OTHER SPECIAL SERrlCE 00 g 0 IA;.,lllAY f'ICK-UPOI SATUR, (Extra charge appties Received FedEx Employee Number Ong1n Agent. Chflrge PART #2041738901 FEC-S-151-1000 REVISION DATE < 2185 'ti) PRINTED U.S.A. Q) ~r~----5~$ ~i~r~·-(\-8~ ~ ..,./ _(,!} ~ G> ~ ~ ~ ~ -~ .... '(i) ~ ? cO ....l (/) Q) ~ ts ~ § \""") 8 p p..~ "° ~ ~ ~ Ul .. i .2'8 ~·s ~ . I Q !;I) .... f5 it ts ~ ~ -~ ----
POST OFFICE BOX 9960 I I 0 I TEXAS A VENUE
COLLEGE ST A TION, TEXAS 77840-2499
January 10, 1986
INTEROFFICE MEMORANDUM
TO:
FROM: (d-~·
SUBJECT :
David J . Pullen, City En gineer
John R. Black, Traffic En gineer
Driveway Access Proposed by Dalsan Properties on Texas
Avenue just South of SH 30
I have reviewed the site plan for a proposed retail center from Dalsan
Properties and understand the owner 1 s concern for driveway access.
Please consider the attached which replaces the two drives proposed
with a single drive located as far as possible from the traffic signal
at SH 30. The 125 foot taper, 75 foot storage lane and 50 foot radius
shown should provide better ingress to this site for southbound traffic
since the driveway intersects Texas Avenue at 60 rather than 90 degrees .
I do not believe it is practical to construct two one-way drives in
this 200 foot section and that the available frontage is best utilized
to im prove right-turn access .
----~ ----
--r
I
DATE o4·o??·t::(p
COMM. NO. ~~4
WE ARE SENDING YOU ~ h 0 f'l [
HEREWITH THE FOLLOWING --\ et'IU~ ~X'f'\"~ DATE CY'() . \Co . t:;(.p
COPIES
URBAN ARCHITECTURE-DALLAS
URBAN ARCHITECTURE-DALLAS 14850 QUORUM DRIVE SUITE 210 DALLAS. TEXAS 75240 214.960.6620
PF,Z
CIT Y OF CO LLE GE STATI ON
APPLICATION FORM
SI TE PLAN REV I E\.J Date /-24-80
NAME oF PROJECT cfol~e ~lzc/o-,.., £efa;/ C6<..-/cr-
LOCAT10N/LEGAL DEscRtPTtoN 12'(.a.> Aue cf &c; "3-e
CAS[ uo.86-40/
0 PRC 1/2q
D Residential
0Comrne rcia1
APPLICANT Utf f-sal'\., 'Prorer--f {~ PHONE (Zft{) q 87-'3ZfZ
ADDRESS 'P-0, Eo~ Z>~70 Vt:il(ttS ~
OWNER 12.ofo er-+ 4 {,Ua_~ I'\ C ~()}~ ~ PHONE {zfc/) tf 87-3Zf 'L-
ARCH 1 TECT OR rnG 1 NEER (Jvbun Acchr'/ec 1-vre PHONE
ADDREss !IB>o Quorum. #z/o Va/las, Teva.> 7~z4o
vef""fj i31S:~p t Ass OCt CA...fc:s I g l z.. We( i;.k_ <:c1 rfe tzo e <;;.
PRESENT USE OF PROPERTY --------------------------------------------------
CURRENT ZONING OF PROP ERTY C-1 ------'------------------------------------------
VAR I AN CE ( S) REQUESTED AND REASON(S) ----------------------------------------
NUMBER OF PARK! NG SPACE S REQU IR ED ___ _
NUMBER OF PARKING SPACES PROVIDED 147 + 3 lu:1nJ,·c4p
The applicant certi fies that the above information i ~ ~ct.
Signatu of owner or agent or applicant
IF APPLICATION IS FILED BY AN YON E OTHER THAN THE OWNER , A POWER OF ATTORNEY STATEMENT
HUST BE INCLUDED.
City of College Station
POST OFFICE BOX 9960 1 10 1 TEXAS A VENUE
COLLEGE STATION. TEXAS 77840-2499
January 30, 1986
MEMORANDUM
TO: Robert Nash & Wayne Nash, Dalsan Properties
Earl Havel, Jerry Bishop & Associates
FROM: Project Review Committee: \ ••
Al Mayo, Director of Planning ~
David Pullen, City Engineer
Walter Wendler, P&Z Representative
Others attending:
Bob Epps, Ass't . Director of Public Services
Joe Guidry, Electrical Superintendent
Jane Kee, Zoning Official
Kim Johnson, Ass't Zoning Official
Jimmy McCord, McCord Engineering
Lavern Dube, GTE
Shirley Volk, Planning Technician
SUBJECT: Parking Lot Plan -Revised -College Station Retail Center
located @Texas Ave & Hwy 30, on south side of
Park Place (86-401)
The P.R.C. met on January 29th to review the revised site plan for the above
mentioned project, and recommends approval of the plan with the following
cond It Ions;'•:
1.
2.
3'
6.
7.
8.
9.
10.
All Islands ~ust be a minimum of 9 feet in width.
Maintain the 8 foot (landscaped ares) setback from the curb of the
deceleration lane.
There may be problems with location/relocation of utll lty 1 ines in
the area of the deceleration lane. Furnish a plan to all utility
entitles concerned which shows existing poles/1 ines.
The State Highway Department has ultimate approval on driveways from
highways. A driveway permit from the Highway Department will be required.
Furnish the total square footage of the entire building, along with the
proposed number of seats In the restaurant for review by the Director
of Planning.
Furnish an elevation of the proposed tower. If signage is applied to
this unoccupied tower, rt will be considered the freestanding sign
allowed for the project by ordinance ,
Number and location of dumpsters has been approved,
The Developer should be aware that there is always a possibll lty that
channelization may · be done on the highways at this location in the
future, which would prevent left-turn movements into or out of this
project.
Check with the Building Department regarding the number of handicapped
parking spaces required.
RE: Landscaping
a. Point totals on this revision are inadequate. Correct on revised plan.
P.R.C . (86-401)
College Station Retail Center (Revision)
Page 2
b. Show hose bibs or include a note regarding irrigation.
c. Groundcover Is required In parking lot Islands, in swales and
drainage areas, In the 8 foot setback area provided by
Section 7.D.7 and in all unpaved portions of street or highway
rtght-of-way abutting the property.
d. Eroded or otherwise displaced soil must be removed from the
right-of-way abutting the development before the Certificate of
Occupancy will be Issued. (Ord. 1505)
SUBMIT 5 COPIES OF REVISED PLAN TO PLANNING DEPARTMENT FOR APPROVAL PRIOR
TO ISSUANCE OF CERTIFICATE OF OCCUPANCY.
sjv
..
CITY OF COLLEGE STATION
P 0 . BOX 9%0 110! TEXAS AVENUE
COLLEGE STATION. TEXAS 77842-0960
Dalsan Properties CS II
14785 Preston Rd. #550
Dallas, Tx. 75240
To Whom It May Concern:
(409) 764-3500
October 9, 1989
As a College Station resident, business person or property
owner~ we know that you are directly concerned with problems
that affect our community. City Ordinances provide that the
owner and/or occupant is responsible for maintaining his/her
property in a safe, attractive and sanitary condition. The
following violation of City Ordinances was noted in a recent
inspection of:
Park Plaza Shopping Center -1800 Texas Ave. S.
__ Land Use/Development Requirements -
Zoning Ordinance 1638.
__ Signs -
Section 12 Ordinance 1638 ~~~~~~~~~~~~~~~~~ ---
__lQL_Landscaping -According to approved minimum landscape
plan on file, this site must replace the following dead or
missing plants: (22) 5-gallon shrubs, (l) 3.5" caliper live
oak, and (9) 1.5" live oak trees within 45 days of receipt
of this notice. Section 11.5 H., Ordinance 1638
This property will be reinspected on November 30, 1989 to
determine compliance with applicable city ordinances.
Please be advised that failure to correct the indicated
violation(s) may result in complaints being filed in
Municipal Court. Fines for conviction are $25.00 to $200.00
for each separate day of violation.
Home of Texas A f., M. ()niversity
We hope that you will join us in our effort to enhance and
improve the beauty and liveability of College Station. If
you have questions or concerns regarding this matter, please
call me at 764-3570.
Sincerely,
Kirn J nson
Planning Assistant
. cc: Legal Division
("
------------·-----·' ... _ -· ·-·-·-· ...... ·---·------------------
Two Allen Center, Suite 2640 Houston. Texas 77002 713-7!)/-1120
FINDINGS
ANALYSIS OF TEXAS AVENUE ACCESS
Kapchinski Development
February 1, 1984
1. Existing Level of Traffic Congestion (Texas Avenue/S.H. 30):
Level of Service E = 70 to 100 percent orobability of signal delays
9reater than one cycle.
2. PM Peak Hour Site Traffic Generation -120 vehicles in/120 vehicles out.
Portion of Site Traffic Using Texas Avenue Access for right turns -10
vehicles in/20 vehicles out.
3. Gaps Availabl~ in PM Peak Period Traffic {Surveyed Monday, January 23, 1984)
Left-Turn Inbound Gaps -< 200 per hour
Left-Turn Outbound Gaps -< 30 per hour
Right-Turn Outbound Gaps -< 200 per hour
4. Driveway Spacing Standards
150 foot separation between driveways (near edge to near edge) to
minimize overlapping conflicts.
250 foot separation from Te~s Avenue/S.H. 30 for left turns.
500 foot separation between .roadway merge point and downstream
driveway entrance.
CONCLUSIONS
1. left turn outbound access from the site onto Texas Avenue between S.H. 30
and Park Place is not feasible for the following reasons:
a) The S.H. 30/Texas Avenue intersection is presently very congested
during peak periods .
b) An inadequate number of gaps in the Texas Avenue traffic stream
are available for left turn exit traffic from the site.
2. Southbound Texas Avenue traffic and left turn traffic from S.H. 30 begin t o
merge about 220 feet north of the propos-ed left turn access driveway. Field
observations confinn that this merging activity (and the turbulent traffic
flow which results) continues beyond this driveway location due to the
difference in speed between these two traffic streams. Adequate gaps are
available in southbound flow for left turns into the site. Thus, left turn
inbound movement could be provided for the site.
[b]
t
Barton-Aschman Associates, Inc.
ANALYSIS OF TEXAS AVENUE ACCESS
February 1, 1984
Page 2
CONCLUSIONS (Continued)
3. There are an adequate number of gaps available in southbound Texas
Avenue traffic to allow for one right-turn-in right-turn-out ac~ess
point for the site. This access drive must be located near the northern
boundary of the site to preclude low speed left turning vehicles from
S.H. 30 from attempting to weave across the relatively high speed
southbound Texas Avenue traffic stream to reach that access point.
I \ ' ' : !
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