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Most highways intersect at grade. To minimize the resulting conflicts
and to provide adequately for the anticipated crossings and turning
movements, the geometric design of the intersection at grade must be
given careful consideration.
Although intersections have many common factors, they are not subject
to a set treatment, and must be looked upon as individual problems.
In varying degrees, four basic factors enter into the design of an
intersection. These factors are traffic, physical, economic and human.
Traffic factors to be considered include: possible and practical
capacities, turning movements, size and operating characteristics of
vehicles, control of movements at points of intersection, vehicle speeds,
pedestrian movements, transit operations, and accident experience.
Physical factors which control intersection design and application
of channelization are: topography, abutting land use, geometric features
of the intersecting roadways, traffic control devices, and safety features.
Economic factors, which are important and often controlling,
include the cost of improvements and the economic effect on abutting
businesses where channelization restricts or prohibits certain vehicular
movements within the intersection area.
Human factors such as driving habits, ability of drivers to
make decisions, effect of surprise, decision and reaction times, and
natural paths of movements must be considered.
An intersection may be extremely simple, or highly developed depending
on the proper evaluation of the foregoing factors. In the redesign of
an existing intersection, standards sometimes must be compromised due
to the high cost of existing development or to the necessity of meeting
rigid physical controls. In the design of a new intersection, however,
such controls frequently can be avoided by a shift in line or grade
of one or both of the intersecting highways.
6.02.1 Capacity Analysis 
Capacity analysis is one of the most important considerations in the
design of intersections. This is especially true in the design of at-grade
intersections on urban streets and highways. Optimum capacities can
be obtained when intersections include auxiliary lanes, proper use of
channelization, and traffic control devices. Reference is made to the
Highway Capacity Manual, Transportation
Research Board, 2000, for procedures in performing capacity computations.
6.02.2 Spacing
The spacing of intersections on major arterials is important to the
capacity and safety of the roadway. In urban areas, the capacity of
the arterial is determined by the capacity of the signalized intersections
along the roadway. Ideally, signalized intersections should be located
no closer than 1200 feet apart. In rural areas, the minimum spacing
of intersections should be one half mile.
6.02.3 Alignment and Profile
Intersections are points of conflict between vehicles, and between vehicles
and pedestrians. The alignment and grade of the intersecting roads should
permit drivers to discern and perform readily the maneuvers necessary
to pass through the intersection safely and with minimum interference
between vehicles. To these ends, the horizontal alignment should be
as straight as possible and gradient as flat as practical. The sight
distance should be equal to or greater than the minimum values for the
specific intersection conditions. Sight distance is discussed later
in this section.
- Alignment
Regardless of the type of intersection, intersecting highways should
meet at or nearly at right angles. Roads intersecting at acute angles
require extensive turning roadway areas. Intersection angles less
than 60 degrees normally warrant realignment closer to 90 degrees.
Intersections on sharp curves should be avoided wherever possible
because the superelevation and widening of pavements on curves complicate
the intersection design. Furthermore, since traffic stripes are not
normally carried through the intersection, there is no visual reference
for the guidance of the driver through the intersection curve during
adverse weather and visibility conditions.
- Profile
Combinations of profile lines that make vehicle control difficult
should be avoided. Substantial grade changes should be avoided at
intersections, although it is not always feasible to do so. Adequate
sight distance should be provided along both highways and across corners,
even where one or both intersecting highways are on vertical curves.
The grades of intersecting highways should be as flat as practical
on those sections that are to be used for storage space for stopped
vehicles. A minimum storage space for 2 vehicles, approximately
50 feet, should be provided for minor streets where stop sign control
is employed and the approach grade is up towards the intersection.
Such slopes should desirably be less than one percent and no more
than 3 percent.
The profile lines and cross sections on the intersection legs should
be adjusted for a distance back from the intersection proper to
provide a smooth junction and proper drainage. Normally, the profile
line of the major highway should be carried through the intersection,
and that of the cross road adjusted to it. Intersections with a
minor road crossing a multi-lane divided highway with narrow median
and superelevated curve should be avoided whenever possible because
of the difficulty in adjusting grades to provide a suitable crossing.
Profile lines of separate turning roadways should be designed to
fit the cross slopes and longitudinal grades of the intersection
legs.
As a rule, the horizontal and vertical alignment are subject to greater
restrictions 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 vehicle operators at all times and definitely
understandable for any desired direction of travel, free from sudden
appearance of potential hazards, and consistent with the portions of
the highway just traveled.
6.02.4 Cross Section
The cross section of the pavement surface within an intersection should
be reviewed on a case-by-case basis. The development of the centerline
profiles and edge of pavement profiles should flow smoothly through
the intersection.
6.03.1 General
There must be unobstructed sight along both roads at an intersection
and across their included corner for distances sufficient to allow the
operators of vehicles approaching the intersection or stopped at the
intersection to carry out whatever maneuvers may be required to negotiate
the intersection.
Any object within the sight triangle high enough above the elevation
of the adjacent roadways to constitute a sight obstruction should be
removed or lowered. Such objects include but are not limited to cut
slopes, hedges, bushes, tall crops, signs, buildings, parked vehicles,
etc. Also check the vertical curve on the highway to see if it obscures
the line of sight from the driver's eye (3.5 feet above the road) to
the approaching vehicle (3.5 feet above the road) as per the sight distance
determined in the next three sections.
6.03.2 Stop Control on Cross Street
Intersection designs should provide sufficient sight distances to avoid
potential conflicts between vehicles turning onto or crossing a highway
from a stopped position and vehicles on the through highway operating
at the design speed.
As a minimum stopping sight distance must be provided. However, to
enhance traffic operations, the recommended sight distance along the
major roadway from Figure
6-A for various design vehicles to turn left, right or cross should
be provided. Where the median width on a divided highway is 6 feet or
greater than the vehicle length, the crossing can be accomplished in
2 steps. The vehicle crosses the first pavement, stops within the median
opening, and proceeds when a safe gap in traffic occurs to cross the
second pavement. However, when the median width is less than that of
a vehicle, the crossing must be made in one step and the median must
be included as part of the roadway width (w).
6.03.3 Yield Control
When an intersection is controlled by a yield sign, the sight triangle
is governed by the design speed on the main highway and that of the
approach highway or ramp.
The approach speed on the yield controlled approach is assumed to be
10 mph for ramps and minor roads. Where two major highways intersect
and one leg is controlled by a yield sign, the design speed on both
highways should be used in determining the minimum sight triangle.
Figure
6-C illustrates the method for establishing the minimum sight triangle
for yield controlled intersections. As a minimum, stopping sight distance
should be provided.
6.03.4 Sight Distance
at Signalized Intersections
Intersections controlled by traffic signals presumably do not require
sight distance between intersecting traffic flows because the flows
move at separate times. However, drivers should be provided with some
view of the intersecting approaches in case a crossing vehicle violates
the signal indication. In addition, sight distance requirements for
vehicles permitted to turn right on red signal indications must be considered.
Line-of-sight should consider the effect of parked vehicles. As a minimum,
stopping sight distance should be provided.
6.04.1 General
One of the primary concerns of intersection design is to provide adequately
for left and right turning movements. The pavement and roadway widths
of turning roadways at intersections are governed by the volumes of
turning traffic and the types of vehicles to be accommodated.
6.04.2 Design Vehicles
The overall dimensions of the design vehicles considered in
geometric design are shown in Table
2-2 Section 2, "General Design Elements." The minimum turning radius
of these design vehicles is shown in Figures 6-D,
6-E, 6-F,
6-G, 6-H
and 6-I.
These figures should be used as guides in determining the turning radii
at intersections and the widths of turning roadways. The principal dimensions
affecting design are the minimum turning radius and those affecting
the path of the inner rear tire, tread width and wheel base. The paths
shown for the several design vehicles are established by the outer trace
of the front overhang and the path of the inner rear wheel.
Due to the greater usage of the 8.5 foot wide, 48 foot long trailers,
the designer is encouraged to use the WB-62 turning template when designing
new intersections or upgrading existing intersections. However, the
designer is cautioned not to arbitrarily provide for these larger vehicles
in the design of all intersections. For example, if the turning traffic
is almost entirely passenger cars, it may not be cost-effective to design
for large trucks, provided that an occasional large truck can turn by
swinging wide and encroaching on other traffic lanes without disrupting
traffic significantly. When selecting the appropriate design vehicle,
the designer is encouraged to use vehicle classification counts. Also,
the existing land use and/or zoning requirements may be useful in selecting
the appropriate design vehicle. However, selection of the design vehicle
will depend on the designer's judgment after all the conditions have
been analyzed and the effect of the operation of larger vehicles has
been evaluated.
It is very possible that the use of more than one design vehicle may
be appropriate. As an example, the design of one quadrant of the intersection
may warrant the use of a SU truck or passenger vehicle while another
quadrant may warrant the use of the WB-62.
It is further recommended that all interstate and freeway ramp terminals
be designed to accommodate the WB-62 design vehicle.
The use of the WB-62 design vehicle should also be considered when
designing ingress and egress to commercial or industrial buildings along
the state highways.
6.04.3 Minimum Edge of
Pavement Design for Turns
Where it is necessary to provide for turning vehicles within minimum
space and slow speeds (less than 10 mph), as at unchannelized intersections,
the minimum turning paths of the design vehicles apply.
For most simple intersections with angle of turn of 90 degrees or less,
a single circular arc joining the tangent edges of pavement provides
an adequate design. Generally, radii of 15 feet to 25 feet are adequate
for passenger vehicles. Radii of 30 feet or more should be provided
to allow an occasional truck or bus to turn without much encroachment.
Radii of 50 feet or more should be provided where large truck combinations
and buses turn frequently.
When provisions must be made for the larger truck units, and the angle
of turn exceeds 90 degrees, a 3-centered compound curve may be used
in lieu of a single circular arc with a large radius.
Figure
6-J indicates the minimum treatment at unchannelized intersections.
6.05.1 General
Where the inner edges of pavement for right turns at intersections
are designed to accommodate semitrailer combinations, or where the design
permits passenger vehicles to turn at speeds of 15 mph or more, the
pavement area at the intersection may become excessively large for proper
control of traffic. To avoid this condition, a corner island, curbed
or painted, should be provided to form a separate turning roadway.
At-grade intersections having large paved areas, such as those with
large corner radii and those at oblique angle crossings, permit and
encourage undesirable vehicle movements, require long pedestrian crossings,
and have unused pavement areas. Even at a simple intersection, appreciable
areas may exist on which some vehicles can wander from natural and expected
paths. Conflicts may be reduced in extent and intensity by a layout
designed to include islands. For the design of 3-centered curves for
right angle turns with corner islands and oblique angle turns with corner
islands, see A Policy on Geometric Design of Highways and Streets, AASHTO,
2001, Exhibit 9-41 and Exhibit 9-42 respectively.
6.05.2 Islands
An island is a defined area between traffic lanes for control of vehicle
movements. Islands also provide an area for pedestrian refuge and traffic
control devices. Within an intersection, a median or an outer separation
is considered an island. This definition makes evident that an island
is no single physical type; it may range from an area delineated by
curbs to a pavement area marked by paint.
Islands generally are included in intersection design for one or more
of the following purposes:
- Separation of conflicts;
- Control of angle of conflict;
- Reduction in excessive pavement areas;
- Regulation of traffic and indication of proper use of intersection;
- Arrangements to favor a predominant turning movement;
- Protection of pedestrians;
- Protection and storage of turning and crossing vehicles;
- Location of traffic control devices.
Islands generally are either elongated or triangular in shape, and
are situated in areas normally unused as vehicle paths. The dimensions
depend on the particular intersection design. Islands should be located
and designed to offer little hazard to vehicles, be relatively inexpensive
to build and maintain, and occupy a minimum of roadway space but yet
be commanding enough that motorists will not drive over them. Island
details depend on particular conditions and should be designed to conform
to the general principles that follow.
Curbed islands are sometimes difficult to see at night because of the
glare from oncoming headlights or from distant luminaries or roadside
businesses. Accordingly, where curbed islands are used, the intersection
should have fixed-source lighting.
When various intersections are involved in a given project and the
warrants are sufficiently similar, a common geometric design for each
intersection should be used. This design approach will enhance driver
expectancy. The designer should also refer to Part V of the Manual on
Uniform Traffic Control Devices for Streets and Highways (MUTCD)
for guidance.
Painted, flush medians and islands may be preferred to the curbed type
under certain conditions including the following: in lightly developed
areas; at intersections where approach speeds are relatively high; where
there is little pedestrian traffic; where fixed-source lighting is not
provided; and where signals, signs, or lighting standards are not needed
on the median or island.
Islands may be grouped into 3 major functional classes: (1) channelizing
islands designed to control and direct traffic movement, usually turning,
(2) divisional islands designed to divide opposing or same-direction
traffic streams, usually through movements, and (3) refuge islands to
provide refuge for pedestrians. Most islands combine 2 or all of these
functions.
- Size
Island sizes and shapes vary materially from one intersection to another.
Islands should be large enough to command attention. The smallest
curbed island that normally should be considered is one that has an
area of approximately 50 square feet for urban streets, and 75 square
feet for rural intersections. However 100 square feet is the minimum
desirable size for islands used in both urban and rural areas.
- Approach-End Treatment
The approach end of a curbed island should be conspicuous to approaching
drivers and should be physically and visually clear of vehicle paths,
so that drivers will not veer from the island.
The nose offset should be 3 feet from the normal edge of through
lane. Figure
6-K shows the recommended design details of curbed triangular
islands under conditions of no shoulder on the approach roadways.
On highways with auxiliary lanes or shoulders, the corner islands
should be offset the full auxiliary lane or shoulder width on both
the main highway and the cross road as shown in Figure
6-L.
- Divisional Islands
The most common type of elongated divisional island is the median
island, for which a design guide is illustrated on Figure
6-M.
6.05.3 Auxiliary Lanes
Auxiliary lanes at intersections serve a wide range of purposes including
space for deceleration and acceleration, bus stops, increased capacity
through an intersection, and storage for turning vehicles. The width
of the auxiliary lanes shall be in accordance with Section
5.03.
Deceleration lanes are always advantageous, particularly on high speed
roads, because the driver of a vehicle leaving the highway has no choice
but to slow down on the through-traffic lane if a deceleration lane
is not provided. On the other hand, acceleration lanes are not always
necessary at stop controlled intersections where entering drivers can
wait for an opportunity to merge without disrupting through traffic.
Acceleration lanes are advantageous on roads with yield control and
on all high volume roads even with stop control where openings between
vehicles in the peak-hour traffic streams are infrequent and short.
When practical, an acceleration or deceleration lane should be of sufficient
width and length to enable a driver to maneuver a vehicle onto it properly
and once onto it, to make the necessary change between the speed of
operation on the highway or street and the lower speed on the turning
roadway. See Figure
6-O for desirable lengths of acceleration and deceleration lanes.
The capacity of a signalized intersection may be increased by adding
an auxiliary lane to accommodate through traffic. The introduction of
the auxiliary lane can usually be accomplished easily since it is effectively
metered into the auxiliary lane. The merging of traffic from the auxiliary
lane back into the through lane beyond the signal requires the auxiliary
lane to be carried a distance beyond the stop line at the traffic signal
to a point where the merging taper is introduced. The minimum length
of the auxiliary lane in advance of and beyond the intersection including
tapers shall be in accordance with Figure
6-N. The Bureau
of Traffic Signal and Safety Engineering must approve the addition
of an auxiliary lane to improve capacity at signalized intersections.
6.05.4 Median Openings
Median openings on divided roadways are provided to permit intended
movements only. Figures 6-P
and 6-Q
show application of grass median and concrete barrier curb median openings,
respectively, to control the various types of movements along a divided
roadway.
The length of the median opening desirably should equal the full roadway
width of the cross road, shoulder to shoulder plus 5 feet on both sides
and in no case less than 40 feet. The control radius (R1)
should also be considered in determining the minimum length of median
opening. The control radius (R1) is determined by the design
vehicle as follows:
| Design
Vehicle |
Control
Radius |
| P and SU |
40 ft. |
| SU, BUS, WB-40 |
50 ft. |
| WB-40, WB-50 |
75 ft. |
Provisions shall be made where pedestrian traffic is present at median
openings, see Figures 6-P
and 6-Q.
The use of a 40 ft. minimum length of opening without regard to the
width of median, the cross road width, pedestrian traffic or the control
radius should not be considered The 40 ft. minimum length of opening
does not apply to openings for U-turns except at very minor crossroads.
Consult A Policy on Geometric Design of Highways and Streets,
AASHTO,
2001, for the design of U-turn median openings.
On urban divided roadways, median openings for U-turns should not be
provided. U-turn movements may be permitted at signalized intersections
where there is sufficient pavement width to accommodate the movement.
Provisions for U-turns should be made on rural divided roadways where
intersections are spaced in excess of one half mile apart. Median widths
in such cases should be at least 20 feet and desirably 30 feet to provide
adequate protection for the vehicle executing the U-turn movement from
the median. It is highly desirable to construct a median left-turn lane
in advance of the U-turn opening to eliminate stopping on the through
lanes.
6.05.5 Median Openings
for Emergency Vehicles
Although it is desirable to require all U-turns by official vehicles
to be accomplished at intersections or interchanges, experience demonstrates
that some emergency median openings are necessary for proper law enforcement,
fire-fighting apparatus, ambulances and maintenance activities. Where
median openings are provided for use by official vehicles only, they
shall be limited in number and carefully located in accordance with
this section and the needs of local authorities.
On freeways and Interstate highways where the spacing of interchanges
is greater than approximately 3 miles, a U-turn median opening may be
provided at a favorable location halfway between the interchanges. Where
the spacing of interchanges is greater than about 6 miles, U-turn median
openings may be provided so that the distance between such openings
or interchange is not greater than about 3 miles.
In general, U-turn median openings should not be provided on urban
freeways due to the close spacing of interchanges. Due to the close
proximity of intersections on divided arterials, emergency U-turn median
openings are not normally provided. However when emergency facilities
are located between intersections, there may be a need for direct access
to the highway.
See Figures 6-R
& 6-S
for typical emergency median opening treatments.
6.06.1 General
A median lane is provided at an intersection as a deceleration and storage
lane for vehicles turning left to leave the highway. Median lanes may
be provided at intersections and other median openings where there is
a high volume of left-turns, or where vehicular speeds are high on the
main roadway. Median lanes may be operated with traffic signal control,
with stop signs, or without either, as traffic conditions warrant.
Figure
6-T shows a typical median left-turn lane.
6.06.2 Lane Width
Left-turn lanes with median curbing should be 11 feet wide and desirably
14 feet wide. The lane width is measured from the curb face to the edge
of through lane. Left-turn lanes without median curbing should be at
least 11 feet wide and preferably 12 feet wide.
Median widths of 20 feet to 25 feet or more are desirable at intersections
with a single left-turn lane, but widths of 15 feet to 18 feet are acceptable.
6.06.3 Length
The total length of the left-turn lane is the sum of storage length
and entering taper.
- Storage Length
The median left-turn lane should be sufficiently long to store the
number of vehicles likely to accumulate during a critical period.
The storage length should be liberal to avoid the possibility of left-turn
vehicles stopping in the through lanes.(see Figure
6-T).
- Taper
The entering taper treatment is illustrated in Figure
6-T.
6.07.1 General
A continuous two-way left-turn median lane provides a common space for
speed changes and storage for left-turn vehicles traveling in either
direction and allows turning movements at any location along a two-way
roadway.
Continuous two-way left-turn median lanes are an effective means of
providing an increased level of service on many urban arterials. They
are especially effective in locations of strip commercial development
and frequent driveway openings experiencing moderate left-turn demands.
Since it is possible for vehicles traveling in opposite directions
to enter the two-way left-turn lane simultaneously, sufficient stopping
sight distance must be provided to permit vehicles to stop.
Table 6-1 provides the desirable and minimum
stopping sight distance as related to design speeds that are applicable
to two-way left-turn lanes.
Table 6-1
Stopping Sight Distance for Two-way Left-Turn Lanes
Design
Speed
(mph) |
Stopping
Sight Distance
(feet) |
30
35
40
45
50
55
60
|
400
500
610
720
850
990
1140
|
The length of crest vertical curve can be computed by the following
formulas. The formulas are based on the height of the driver's eyes
of 3.5 feet and of an object 2 feet on the road, which is equivalent
to the headlight height above the roadway.
When S is greater than L, L=2S-(2158/A)
When S is less than L, L=AS2/2158
S= Stopping sight distance from Table 6-1, in feet.
L= Length of vertical curve, in feet.
A= Algebraic difference in grade, in percent.
If there is adequate roadway lighting present, the object height may
be increased to 4.25 feet (top of vehicle), therefore, substitute "3093"
for "2158" in the previous formulas. The vertical curve length on the
highway should also be checked by Figure
4-I and use the greater of the two "L" values when designing the
vertical curve.
Figure
6-U shows a typical two-way left-turn median lane.
6.07.2 Lane Width
Lane widths for continuous two-way left turn median lanes range from
12 feet to 16 feet. The wider pavement width should be used only when
raised islands are provided at major intersections with high left-turn
demands. A median lane width of 12 feet is desirable where raised islands
are not provided at major intersections.
6.07.3 Cross Slope
Generally the crown line should be located in the center of the median
turn lane. The slope of pavement from the crown line should be the same
as the cross slope on the through lane adjacent to the median lane.
6.08.1 General
A "jughandle" is an at-grade ramp provided at or between intersections
to permit the motorists to make indirect left turns and/or U-turns.
Around-the-block designs that use interconnecting local street patterns
to accomplish indirect left turns or U-turns are not considered "jughandles".
These ramps exit from the right lane of the highway in advance of the
intersection, or past the intersection and convey traffic across the
main highway under traffic signal control. This movement eliminates
all turns within active traffic lanes and, in addition to providing
greater safety, reduces delays to the through traffic that left turning
vehicles usually create.
6.08.2 Ramp Width
Ramp widths are based on Figure
7-B. The minimum width for a one lane ramp should not be less than
22 feet. Ramps may have more than one lane when greater traffic volumes
are anticipated.
6.08.3 Access Control
In order to provide safe and efficient traffic operations on land service
highways, the interior of all jughandles shall be acquired. In addition,
no access is permitted on the outside of all jughandles including the
entire length of acceleration and deceleration lanes, excluding the
taper length, see Figures 6-V,
6-W
and 6-X.
It is desirable that no access is permitted along the taper length of
acceleration and deceleration lanes.
When a deceleration lane extends through an intersection and the deceleration
lane accommodates both the right turn move onto the cross street and
the right turn onto the jughandle past the intersection; the access
restriction that applies in advance of the intersection is "corner clearance",
see Figure
6-X. The deceleration lane following the intersection has no access
permitted in accordance with the prior paragraph.
Where access is proposed at new or existing jughandle locations, design
waivers (submitted as an attachment to the permit application) to the
above paragraph will be granted only after a thorough analysis has been
made with respect to the cost of acquisition and impact on safety. For
further information on access control, see Section
5.08, "Driveways"
6.08.4 Standard Jughandle
Designs
Figures 6-V,
6-W
and 6-X
illustrate the three basic jughandle configurations. The dimensions
and radii shown are recommended, however, social, environmental or economic
impacts may make adherence to the basic geometrics impractical.
The recommended design speeds for the basic
jughandle configurations are shown in Table 6-2.
Table 6-2
Jughandle Design Speeds
| Jughandle
Type |
Minimum
Design
Speed
(mph) |
| A |
25 |
| B - one lane |
15 |
| B - one lane with T intersection |
20 |
| B - two lane |
25 |
| C - loop ramp |
15
(20 des.) |
| C - finger ramp |
25 |
When initially providing jughandles at locations where there are no
existing cross streets or there is an intersecting street on only one
side, the designer should evaluate the future development potential
of the property adjacent to the jughandle. Consideration should be given
to designing the jughandle for future expansion to accommodate the access
needs of the adjacent property.
The design of Type "B" jughandles should generally be limited to locations
where the development of the adjacent land is limited due to topography,
environmental constraints, zoning restrictions, etc.
6.08.5 Superelevation
and Cross Slope
It is desirable to provide as much superelevation as practical on jughandles,
particularly where the ramp curve is sharp and on a downgrade.
Table 6-3 provides a suggested range of superelevation
rates in percent for various ramp radii. Rates in the upper half or
third of the indicated range are preferred. The cross slope on tangent
sections of ramps is normally sloped one-way at 2 percent, which is
considered a practical minimum for effective drainage across the surface
(see Figure
5-J).
Table 6-3
Jughandle(Ramp) Superelevation(%)
Design
Speed
(mph) |
Radius
(feet) |
| 50 |
90 |
150 |
230 |
310 |
430 |
| Superelevation
(%) |
| 15 |
2-6 |
2-6 |
2-5 |
2-4 |
2-3 |
2-3 |
| 20 |
--- |
2-6 |
2-6 |
2-6 |
2-4 |
2-3 |
| 25 |
--- |
--- |
4-6 |
3-6 |
3-6 |
3-5 |
| 30 |
--- |
--- |
--- |
6 |
5-6 |
4-6 |
Exceptions to the use of full superelevation are at street intersections
where a stop or yield condition is in effect.
The length of superelevation transition should be based on Section
4.03.2.2. With respect to the beginning and ending of a curve on
the ramp proper (not including terminals), see Table
4-4 for the portion of the runoff located prior to the curve. This
may be altered as required to adjust for flat spots or unsightly sags
and humps when alignment is tight. The principal criteria is the development
of smooth-edge profiles that do not appear distorted to the driver.
See Section
7.06.2, "Ramp Terminals", , for a discussion on development of superelevation
at free-flow ramp terminals and the maximum algebraic difference in
cross slope at crossover line.
6.09.1 Parking Restrictions
At Intersections
Vehicular parking should not be permitted within the immediate limits
of at-grade intersections; see Section
6.03, "Sight Distance" for sight distance requirements
at intersections.
6.09.2 Lighting At Intersections
Lighting affects the safety of highway and street intersections and
the ease and comfort of traffic operations. In urban and suburban areas
where there are concentrations of pedestrians and roadside and intersection
interference, fixed-source lighting tends to reduce accidents. Whether
or not rural at-grade intersections should be lighted depends on the
planned geometrics and the turning traffic volumes involved. Intersections
that generally do not require channelization are seldom lighted. However,
for the benefit of non-local highway users, lighting at rural intersections
is desirable to aid the driver in ascertaining sign messages during
non-daylight period.
Intersections with channelization, particularly with multiple-road
geometrics, should include lighting. Large channelized intersections
especially need illumination because of the higher range of turning
radii that are not within the lateral range of vehicular headlight beams.
Vehicles approaching the intersection also must reduce speed. The indication
of this need should be definite and visible at a distance from the intersection
that may be beyond the range of headlights. Illumination of at-grade
intersections with fixed-source lighting accomplishes this need.
See Section 11, "Highway
Lighting Sections,", for guidelines in the planning and design of
roadway lighting systems.
6.10.1 Introduction
To reduce conflicts on state highways between through traffic and buses
stopped to receive and/or discharge passengers, bus turnouts may be
provided on landservice highways when the outside shoulders on the highway
are less than 10 feet in width. The designer shall in conjunction with
the transit agency providing service along the highway determine the
locations of the bus turnouts.
6.10.2 Location Criteria
When it has been determined that bus turnouts are warranted, the following
criteria should be used to select the bus turnout location(s):
- Desirably there should not be any driveways within the bus turnout.
As a minimum, there shall be no driveways located within the bus stopping
area. Driveways may be located within the acceleration and deceleration
portion of the bus turnout including the taper. However to minimize
conflicts between the vehicles using the driveway and the bus, the
bus stopping area should desirably be located on the far side of the
driveway.
- The vertical and horizontal highway geometry meets current stopping
sight distance criteria.
- The bus turnouts should be located where patrons may park and cross
roadways legally and safely. Desirably bus turnouts should be located
within 400 feet of an intersection or parking areas used by the bus
patrons. Alternatives including review and possible modification of
parking regulations may be considered.
- There is sufficient right-of-way available, or if required, would
not involve the acquisition of developed parcels or environmentally
sensitive parcels.
- The bus stop is close to the points of origins and/or destinations
of the transit rider. Locations convenient to park "n" ride facilities,
intermodal transfer facilities and transfer facilities between bus
services are desirable.
- Access to and from the bus stop is convenient to well lit pedestrian
crossings, crosswalks or signalized crossings.
- A bus turnout may be placed on the far side or near side of an intersection,
or at mid-block. When placed at intersections, locating the bus turnout
on the far side is preferred. Near side bus turnouts create conflicts
with right turning traffic, obscures pedestrian view of oncoming traffic,
and may obscure a driver’s view of signs, traffic control devices
and pedestrians. Mid-block turnouts may be provided when there is
a need to service a major pedestrian traffic generator (i.e. shopping
mall, school, railroad station, hospital, etc.).
- The location of the bus turnouts conform to local ordinances.
6.10.3 Other Considerations
In addition to the location criteria noted in Section 6.10.2, the following
features should be considered when selecting bus turnout locations:
- Utility and signal poles - The relocation of utility poles
could require the acquisition of additional right-of-way, and depending
upon the type of service provided involve excessive relocation costs.
The location of the bus turnouts at intersections could involve costly
signal relocations and when placed on the near side of the intersection
stopped buses could obscure the signals.
- Drainage - To avoid splashing of bus riders turnouts should
not be located at low points in the vertical alignment. Also, additional
inlets may be necessary to limit the spread in the gutter to 3 feet.
Grades should be checked to avoid ponding where pavement cross slope
exceeds the longitudinal slope in the turnout.
- Guide rail - Openings in guide rail located along the curbline
may not be permitted due to inadequate length of need or the inability
to provide the proper end treatment.
- Signing - The location of the bus turnout could interfere
with the visibility of regulatory, warning and/or directional signs.
The relocation of existing signs and/or the installation of new signs
including bus stop signs shall be coordinated with the Bureau
of Traffic Signals and Safety.
- Handicapped ramps - When the construction of a bus turnout
impacts an existing handicapped ramp(s) at an intersection, the designer
shall assess the entire intersection to determine if the remaining
handicapped ramps will be compatible.
- Curb - Curb shall be provided at all bus turnouts. The curb
height shall conform to Section
5.06.
- The pavement section for widening or reconstruction of shoulders
for bus turnouts should be determined by Geotechnical Engineering.
6.10.4 Bus Turnout Design
Criteria
Figure
6-Y illustrates typical bus turnout designs for a far side and an
alternate far side bus turnout. Figure
6-Z illustrates a typical near side and mid-block bus turnout. When
a far side bus turnout is to be provided, the alternate far side bus
turnout should be considered at intersections where there is a high
volume of traffic making right turns that may use the bus turnout as
an auxiliary lane.
The bus stopping areas shall be a minimum of 50 feet in length for
each standard 40 feet bus and 70 feet for every 60 feet bus expected
to use the bus turnout. When more than one bus is expected to use the
turnout simultaneously, the length of the bus stopping area should be
adjusted accordingly. Desirably the width of the bus stopping area including
the acceleration and deceleration lanes should be 12 feet. Where it
is not practical to provide the 12 feet width, a minimum width of 10
feet may be provided to reduce right-of-way or environmental impacts.
Bus turnouts generally consist of entrance and exit tapers, deceleration
and acceleration lanes, and a bus stopping area. The length of the tapers
and the deceleration and acceleration lanes vary depending on the posted
speed of the highway.
Table 6-4 provides the desirable lengths. The
use of lengths less than those shown in Table 6-4 may cause delays to
the transit service and adversely impact the traffic flow on the highway.
Table 6-4
Acceleration - Deceleration Lengths (Desirable)
Posted
Speed
(mph) |
Length of
Acceleration
Lane
(feet) |
Length of
Deceleration
Lane
(feet) |
Length of
Entrance and
Exit Tapers
(feet) |
| 35 |
250 |
185 |
170 |
| 40 |
400 |
265 |
190 |
| 45 |
700 |
360 |
210 |
| 50 |
975 |
470 |
230 |
| 55 |
1400 |
595 |
250 |
| Source: TCRP Report 19 Guidelines for the Location
and Design of Bus Stops
|
As a minimum bus turnouts may be constructed without acceleration and
deceleration lanes when it is not practical to provide the above lengths.
However, the designer should attempt to provide as much acceleration
and deceleration lane length as practical.
The taper lengths shown in the Table are desirable. Minimum entrance
and exit tapers shown in Figure 6-Y
and 6-Z
may be provided when it is not practical to provide those shown in the
Table. The minimum lengths of taper are applicable with or without acceleration
or deceleration lanes.
The pavement cross slope in the bus turnout shall be one half (1/2)
percent greater than the adjacent through lane. On superelevated roadway
sections, the pavement cross slope shall be the same as the adjacent
through lane. When conditions dictate maintaining drainage flow in the
existing gutter, the bus turnout may be sloped toward the gutter line
at 1.5 to 2.0 percent.
The width of the sidewalk in the bus loading area shall be a minimum
of 7 feet and desirably 12 feet. The width of the sidewalk approaching
the loading area shall conform to Section
5.07.2. Sidewalk shall be provided where there is no existing sidewalk
approaching the bus loading area. A survey of pedestrian traffic should
be used in establishing a reasonable limit for the proposed sidewalk.
Revised as per BDC02MR-3 (12/27/02)
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