Refer
to Chapter XI of the AASHTO Green Book
- Chapter 8 (this has been edited
and several pictures and tables do not appear on this page.
BACKGROUND
At
grade intersections are
one of the most critical and most complicated elements in highway
design.
The efficiency, safety, speed, cost of
operation, and capacity of
the highway system depend on the design of its intersections. Design
criteria
that are used to create the most efficient
roadways are easily thwarted
when that roadway meets up with intersecting traffic vying for the same
limited roadway space. In urban and
suburban areas in particular,
the capacity of signalized intersections can effectively define the
capacity
of the highway system. Add the need
safely to accommodate bicyclists
and pedestrians with varying degrees of mobility, and the need to
handle
left and right turns, and the challenge
faced by designers becomes
even more complicated.
The Basics of Intersection Design
As stated in the AASHTO Green Book, the main objective of intersection design is to:
...reduce
the severity of potential conflicts between motor vehicles, buses,
trucks,
bicycles, pedestrians, and facilities,
while
facilitating the convenience, ease, and comfort of people traversing
the
intersections.(p. 627)
Two kinds of intersections - at
left busy, and at right, designed to limit thru traffic.
INTERSECTION DESIGN ELEMENTS
As
is the case with other aspects
of the highway design process, designers can use a wide range of
intersection
design elements in
combination to provide both
operational quality and safety. These include:
Traffic
islands to separate conflicting vehicle movements
Street
closures or realinements to simplify the number and orientation of
traffic
movements through an intersection
Separate
left and rightturn lanes to remove slowmoving or stopped vehicles from
through traffic lanes
Medians
and channelized islands to provide refuge for pedestrians and
bicyclists
out of the vehicular traveled way.
The following paragraphs summarize of primary intersection design guidelines.
Angle of Intersection
Crossing
roadways should intersect
at 90 degrees, if possible, and at no less than 75 degrees. Skew angles
of 60 degrees or less may need
geometric countermeasures,
such as reconstruction, or traffic control, such as signalization.
Horizontal and Vertical Alinement
The
alinement before and through
an intersection must promote driver awareness, operate well under
frequent
braking, and be easy to drive, so
that the navigational task
is not too difficult. The Green Book has recommended values for the
minimum
stopping sight distance needed based
on the design speed of the
approach roads. The design of intersections should also incorporate
provisions
for intersection sight distance.
Medians
Medians,
either raised or painted,
provide a physical separation between opposing traffic flows. They also
provide a refuge area for
pedestrians to wait at crossing
locations. Medians are a standard form of channelization at rural
roadways
and urban street intersections
carrying four or more lanes.
There are two principal functions of medians specifically located at
intersections:
Separating
opposing traffic flows
Providing
storage for vehicles making left and Uturns and vehicles crossing
traffic
and shielding pedestrians
Another
important benefit of
a median in an urban area is that it offers a green space for trees and
lowgrowing plant material. Careful
consideration is needed, however,
to select the proper location and type of plantings. Particularly in
narrow
medians, plantings can create
maintenance problems, and
trees can cause visual obstructions if not carefully located.
Field
studies and accident
analysis provide similar findings on the operational and safety effects
of the median width at intersections.' At rural
unsignalized intersections,
accidents and undesirable driving behavior decrease as the median width
increases. In contrast, at suburban
signalized and unsignalized
intersections, accidents and undesirable driving behavior increase as
the
median width increases. 1 Median
Intersection Design, NCHRP
Report 375, Transportation Research Board, National Research Council,
National
Academy Press, Washington, DC, 1995.
In
other words, at rural unsignalized
intersections, wider medians are preferable to narrower medians, unless
signalization or suburban
development is anticipated.
At suburban intersections, the median should not be wider than
necessary
to accommodate the median leftturn
treatment needed to serve
current and future traffic volumes.
Left Turn Lane Warrants and Design
Leftturn
lanes may provide
added safety and efficiency at both unsignalized and signalized
intersections.
At signalized intersections, leftturn lane
warrants are based on the
magnitude of turning movements, accident experience, and general
capacity
relationships. The design values for
leftturn approach tapers,
turn bay tapers, and storage lane lengths are based on deceleration in
the lane, storage in the lane, or a combination of
both. At signalized intersections,
the required length of storage bay is a function of signal cycle length.
An
example of a simple safety
improvement is the addition of a painted leftturn lane at a rural
intersection.
This action not only reduces the
potential for yearend accidents,
but also provides drivers with a comfortable way to make a left turn.
However,
as is discussed in the Issues
section of this chapter, the
addition of a left turn lane can also affect resources along the side
of
the road or change the character of the road
corridor. These are tradeoffs
for designers to consider.
RightTurn Lane Warrants and Design
Depending
on rightturn traffic
volumes, accident history, highway speed, and availability of
rightofway,
rightturn lanes may be appropriate for
some intersections. As with
leftturn lanes, the taper and storage length design is based on
deceleration,
storage requirements, or both.
Corner Radius Design
The
design for an intersection
corner radius is based on the selection of a reasonable design vehicle
for the specific location. Design vehicles
can range from large (tractortrailer
combinations) to small (private autos). There are a number of tradeoffs
involved in this decision. Designing
the corner radius for large
vehicles requires more open intersections, and increases cost, and such
intersections are more difficult to mark,
signalize, and operate. In
addition, the larger the dimensions of the radius, the greater the
distance
across the intersection from one side of the
street to the other. This
can make crossing the intersection much more difficult for pedestrians,
particularly people who are elderly or have
mobility impairments. Conversely,
designing the corner radius for small vehicles can create operational
problems
should a significant number of
larger vehicles have to use
the intersection.
The
actual radius or curb return
design can be accomplished in one of four ways. Simple circular radius
designs are the most commonly
encountered design on lowspeed
collector and local streets and in downtown areas. Alternative design
methodologies
include the use of
symmetrical threecentered
compound curves, asymmetrical threecentered compound curves, or simple
radius curves with tapers. These
designs better fit the paths
of turning vehicles, thereby providing more efficient operations.
Traffic Islands
Traffic
islands, or channelization,
represent one of the most important tools in the design of
intersections.
Islands can either be painted directly on
the roadway surface or they
may be raised. Painted or "flush" channelization may be used on
highspeed
highways to delineate turning lanes, in
constrained locations, or
where snow removal is a concern. Raised islands, with appropriate
channels
or curb ramps to accommodate users of
wheelchairs or other related
devices, should be used where the primary function of the island is to
shield pedestrians, locate traffic control
devices, or prohibit undesirable
traffic movements.
There
are two basic types of
traffic islandscorner islands that separate rightturning vehicles and
median
or divisional islands that separate
opposing traffic flows on
an intersection approach. Although islands in general provide a safe
refuge
for pedestrians, corner islands that
separate rightturning vehicles
in particular may make crossing intersections more difficult for
pedestrians.
These islands tend to widen the
crossing distance. They can
also make it more difficult for pedestrians to maneuver through the
intersection,
see oncoming traffic making right
turns, and know where to cross,
if the islands are not clearly delineated.
Traffic Control Devices
Traffic
control devices are
installed to designate rightofway at intersections and to provide for
the
safest and most efficient movement of all
traffic, including pedestrians
and bicyclists. The standards established in the latest edition of the
Manual on Uniform Traffic Control Devices for
Streets and Highways (MUTCD),
published by the FHWA, must be followed to determine proper
intersection
control.
NEW INTERSECTION DESIGN
CONCEPTS
In
recent years, a new intersection
design concept has evolved to provide an alternative to the traditional
T, fourleg, and multileg intersections.
This design concept is called
a roundabout.
Modern
roundabouts are increasingly
being recognized as design alternatives to the use of traditional
traffic
signals for intersections for arterials.
They improve both safety and
efficiency for pedestrians and bicyclists, as well as motor vehicles.
So
far, roundabouts have been built in such
States as California, Colorado,
Maryland, Nevada, Florida, and Vermont. These roundabouts are different
from rotary or traffic circles that have
been used in the United States
for a number of years to give entering traffic the rightofway and
encourage
higher design speeds.
The
modern roundabout is designed
to slow entering traffic and allow all the traffic to flow through the
junction freely and safely. Unlike the older
rotary design, entering vehicles
must yield the rightofway to vehicles already in the circle. A
deflection
at the entrance forces vehicles to slow
down. Traffic signals are
not used, and pedestrians cross the streets at marked crosswalks.
Roundabout.
The
average delay at a roundabout
is estimated to be less than half of that at a typical signalized
intersection.
Decreased delay may mean that
fewer lanes are needed. Signalized
intersections often require multiple approach lanes and multiple
receiving
lanes, which leads to a wider road.
Perhaps
the greatest advantages
of roundabouts are their urban design and aesthetic aspects.
Roundabouts
eliminate the clutter of overhead
wires and signal poles and
allow signage to be reduced. They can be distinctive entry points into
a community or mark a special place. The
central island offers an opportunity
for a variety of landscape designs, as well.
ISSUES
Each
of the various components
of intersection design can cause conflicts between the need for a safe
and adequate design, on the one hand,
and the need to minimize impact
to the surrounding physical and human environments, on the other. In
addition,
the need to accommodate
pedestrians and bicyclists
can sometimes cause conflicts with the need to provide an efficient
operating
environment for vehicular traffic.
Accommodating Pedestrians
The
safe and efficient accommodation
of pedestrians at intersections is equally important as the provisions
made for vehicles. Pedestrian
movements should be provided
for and their locations controlled to maximize safety and minimize
conflicts
with other traffic flows. Too often,
pedestrians are a secondary
consideration in the design of roadways, particularly at intersections
in suburban areas.
Solution
For
all but a few exceptions,
pedestrian crosswalks should be located at intersections, should have
appropriate
curb ramps for accessibility, and
should be clearly marked.
Two parallel painted lines generally are not enough of a distinguishing
marking. Often motorists confuse these lines
with the stopping line and
pull right up to the edge of the crosswalk. At a minimum, some type of
striping or painting inside the crosswalk area is
recommended to improve safety.
Many cities and suburban areas have gone beyond this and added
aesthetic
treatments to their crosswalk
designs, including use of
the following:
Distinguishing
materials for crosswalks, such as brick, patterned concrete, and
cobblestone
Granite
edging
Colored
pavement or solid painting of crosswalks.
Appropriate Corner Radius Design
As
mentioned earlier, there
are many tradeoffs involved in the selection of the appropriate type
and
dimension of radius designs. Issues arise
when all of the factors involved
in the design decision are not considered. For example, if the primary
intent of the intersection design is to move
traffic through as quickly
as possible, a higher corner radius would be selected. The dimensions
of
the corner radius send a message to drivers
entering residential neighborhoods
regarding the speed they can drive and should be designed with this in
mind. Encouraging fast speeds
around intersection corners
into residential areas will undermine efforts to lower operating speeds
within the neighborhoods themselves. In
addition, faster speeds create
an unsafe environment for pedestrians.
Addition of LeftTurn Lanes
A
common conflict arising from
the use of channelization, or separation of traffic into definite paths
of travel by traffic islands, medians, or
pavement markings, is the
addition of leftturn lanes. While there is no doubt that this can
create
a smoother flowing intersection, especially on
twolane roads, the addition
of a leftturn lane can significantly widen the width of the roadway,
unless
there is a median. This can change the
character of an area, affect
adjacent development or resources, and cause the road to be out of
scale
with its surroundings.
Solution
In
cases where a leftturn lane
is truly needed to improve safety and operational efficiency in a
constricted
rightofway, there may not be an easy
solution to this issue. Sometimes
the addition of leftturn lanes depends on new growth and development
along
the corridor. If the scenic, historic,
or cultural resources are
such that any additional widening would affect these resources, it may
be that decisions made at the land use stage of
planning should be reconsidered.
Limiting development along the corridor will limit traffic volumes and
the need for additional leftturn lanes.
Another option is to lower
traffic volumes on the roadway through other means, including creating
or widening alternative routes.
It could be like a junior high dance all over again.
People cautiously circle, not quite sure of what to do next. Finally, one person, then another gets up the courage to merge over to the other side. The result can be either a horrific accident or a smooth exit, finally reaching the goal.
If Oak Harbor and the Washington Department of Transportation install up to six roundabouts along Highway 20 between Swantown Avenue and Cabot Drive, it would do so over time, WSDOT Planning Engineer Eric Shjarback said at a Thursday evening open house.
“There’s a period of adjustment,” Oak Harbor resident Margaret Nichols said. “Sometimes it’s a long period of adjustment.”
Nichols
said she has encountered
roundabouts while on the east coast. She said that people need to be
educated
on the proper etiquette for navigating them. A roundabout is a
traffic
control device that circles traffic in the same direction with multiple
outlets to allow people to turn off.
Oak Harbor resident Jim Campbell
had to learn how to drive in a roundabout while living for two years in
Scotland. He said that the newness of them created a problem for him at
first, but he was soon able to zoom about with the locals.
“There’s nothing worse than being stuck in the middle of a roundabout and not knowing what leg to go out,” Campbell said. The roundabouts are being considered as a means to alleviate the growing traffic congestion problems in Oak Harbor. The study focuses on six intersection along Highway 20 between Swantown Avenue and Cabot Drive.
The city is spending $20,000 and the state is chipping in another $10,000 to find the best possible ways to ease the burden of traffic. Oak Harbor experiences between 17,000 and 20,000 vehicles traveling Highway 20 between Swantown and Cabot Avenue each day. WSDOT planners estimate that number will increase to at least 30,000 vehicles each day by the year 2030.
For the long term, the roundabouts ease congestion for a longer time than traffic signals, Shjarback said. “If we are constrained to two lanes in each direction, the signals end up failing around the year 2020,” he said. Roundabouts have moved their way to the top of the list for a variety of reasons, but tops on the list is safety, Shjarback said. By having people navigate an obstacle at slow speeds, people are more aware of their surroundings.
“The flow of the traffic is neat,” he said. “The traffic just kind of disappears.”