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From Tech Transfer Newsletter, Winter 2006
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Reconsidering Roundabouts
This proven safety solution reduces the number and severity of intersection
crashes and is gaining popularity in California and across the U.S.
An Issue Brief by the Federal Highway Administration and the Institute of
Transportation Engineers
History of Roundabouts
The "modern roundabout" is commonly confused with older-style traffic circles
and rotaries. Traffic circles have been around almost a century, with the first
documented one in the U.S. being built in 1905 on the southwest corner of
Central Park in New York City and named after Christopher Columbus. From the
start, traffic circles provided the ability for a city to tie a number of
intersecting streets together and make a landscaped central circle that had
aesthetic value to the community. Many large circles or rotaries were built in
the United States until the 1950s when they fell out of favor. The older-style
rotaries enabled high-speed merging and weaving of vehicles that led to a high
crash experience.
The modern roundabout was developed in the United Kingdom to rectify problems
associated with these traffic circles. In 1966, the United Kingdom adopted a
mandatory "give-way" rule at all circular intersections, which required entering
traffic to give way, or yield, to circulating traffic. This rule prevented
circular intersections from locking up by not allowing vehicles to enter the
intersection until there were sufficient gaps in circulating traffic.
What is a Modern Roundabout?
A modern roundabout is a one-way circular intersection without traffic signals
in which traffic flows around a center island. Roundabouts feature yield control
for all entering traffic, channelized approaches and appropriate geometric
curvature to ensure that travel speeds on the circulatory roadway are typically
less than 30 mph. Roundabouts must be designed to meet the needs of all users --
drivers, pedestrians, pedestrians with disabilities and bicyclists. When
designing roundabouts, special considerations must be given to the needs of
pedestrians with visual disabilities who are unable to judge adequate gaps in
traffic at roundabouts. Proper site selection and pedestrian channelization are
essential to making roundabouts accessible to all users. Roundabouts can also be
designed for trucks and larger vehicles and in geographic areas where
significant snowfall is the norm during the winter.
Features of Modern Roundabouts
The design and traffic control features of roundabouts are as follows:
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Yield control is used on all entries.
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Circulating vehicles have the right-of-way. All vehicles circulate
counter-clockwise and pass to the right of the central island.
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A central island further channelizes vehicles' paths once within the circulatory
roadway.
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Pedestrian access is allowed only across the legs of the roundabout, behind the
yield line to the circulatory roadway.
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Each approach to the roundabout has a splitter island – a raised or
painted area on an approach used to separate entering from exiting traffic, and
to channelize and slow entering traffic.
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Yield lines mark the point of entry from each approach into the circulatory
roadway. Entering vehicles must yield to any circulating traffic coming from the
left before crossing this line into the circulatory roadway.
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Landscaping buffers are provided at most roundabouts to separate vehicular and
pedestrian traffic, and to encourage pedestrians to cross only at the designated
crossing locations.
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Accessible pedestrian crossings should be provided at all roundabouts. The
crossing location is set back from the yield line and the splitter island is cut
to allow pedestrians, wheelchairs, strollers and bicycles to pass through.
Tactile surfaces should be used to warn pedestrians with visual disabilities
that they are about to enter the roadway.
Roundabout Safety
Research indicates that well-designed roundabouts can be safer and more
efficient than conventional intersections, because:
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Roundabouts have fewer conflict points than conventional intersections.
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Roundabout use eliminates the potential for hazardous conflicts, such as
right-angle and left-turn head-on crashes. Because fewer potential conflicts
between road users are present, and pedestrian crossing distances are shorter,
single-lane approach roundabouts produce greater safety benefits than multilane
approaches.
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Low absolute speeds associated with roundabouts allow drivers more time to react
to potential conflicts and also help to improve the safety performance of
roundabouts.
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Most road users travel at similar speeds through roundabouts. Their lower
relative speeds reduce crash severity compared to some traditionally controlled
intersections where the difference in vehicle speeds is greater.
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Roundabouts have fewer annual injury crashes than rural two-way stop-controlled
intersections, regardless of minor street demand volumes.
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Roundabouts have fewer injury accidents per year than signalized intersections,
particularly in rural areas. At volumes greater than 50,000 average daily
traffic (ADT), urban roundabout safety may be comparable to that of urban
signalized intersections.
A December 2002 report by the Maryland Highway Administration indicates that 15
single-lane roundabouts have greatly improved intersection safety in the area
where they were installed. The analysis shows there has been a 100 percent
decrease in the fatal crash rate; a 60 percent decrease in the total crash rate;
an 82 percent reduction in the injury crash rate; and a 27 percent reduction in
the property damage only accident rate.
Safety Problems Susceptible to Correction by Roundabouts
The decision to install a roundabout as a safety improvement should be based on
a demonstrated safety problem of the type susceptible to correction by a
roundabout. A review of crash reports and the type of accidents occurring is
essential.
Examples of safety problems include:
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High rates of crashes involving conflicts that would tend to be resolved by a
roundabout (right angle, head-on, left/through, U-turns, etc.).
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High-crash severity that could be reduced by the slower speeds associated with
roundabouts.
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Site visibility problems that reduce the effectiveness of stop sign control (in
this case, landscaping of the roundabout needs to be carefully considered).
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Inadequate separation of movements, especially on single-lane approaches.
Issues to Review When Considering Roundabout Design Alternatives
The following issues should be considered before making the decision to
implement a roundabout design:
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The context of the project must be considered. What are the regional policy
constraints that must be addressed? Are there site specific and community impact
reasons why a roundabout of any particular size would not be a good choice?
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The space feasibility for a roundabout. Is there enough right-of-way to build
the roundabout? Is right-of-way acquisition required? If "yes," this introduces
administrative complications that some agencies might want to avoid.
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The physical or geometric complications - such as right-of-way limitations,
utility conflicts, drainage problems and unfavorable topography - that may limit
visibility or complicate construction.
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The proximity of generators of significant traffic that might have difficulty
negotiating the roundabout, such as high volumes of oversized trucks.
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The proximity of traffic control devices that would require preemption, such as
railroad tracks or drawbridges.
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Whether traffic congestion would cause routine back-ups into the roundabout,
such as over-capacity signals or freeway entrance ramps. The successful
operation of a roundabout depends on unimpeded flow on the circulatory roadway.
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Whether nearby intersections of a major arterial and a minor arterial or local
road might create unacceptable delay to the major road. Roundabouts delay and
channelize all traffic entering the intersection and could introduce excessive
delay or speed inconsistencies to flow on the major arterial.
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Heavy pedestrian or bicycle movements in conflict with high traffic volumes pose
problems for all types of traffic control.
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Is there a coordinated signal system? Are the intersections located on arterial
streets within a coordinated signal network? In these situations, a signalized
intersection incorporated into the system might improve the level of service on
the arterial.
The existence of one or more of these conditions does not necessarily preclude
the installation of a roundabout. Roundabouts have been built at locations that
exhibit nearly all of the conditions listed above. The issues may be resolved
through coordination with and support from other agencies and implementation of
specific mitigation actions.
To Learn More
To learn more about how to install Roundabouts that can benefit your community,
sign up for Tech Transfer's course, Roundabout Design (TE-26).
The course will be held June 13-14, 2006 in Richmond, CA. Registration is
available at
www.techtransfer.berkeley.edu/training.
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