From Tech Transfer Newsletter, Winter 2006
This document is found at www.techtransfer.berkeley.edu/newsletter/06-1/roundabout.php

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:

• Yield control is used on all entries.
• Circulating vehicles have the right-of-way. All vehicles circulate counter-clockwise and pass to the right of the central island.
• A central island further channelizes vehicles' paths once within the circulatory roadway.
• Pedestrian access is allowed only across the legs of the roundabout, behind the yield line to the circulatory roadway.
• 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.
• 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.
• 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.
• 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:

• Roundabouts have fewer conflict points than conventional intersections.
• 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.
• 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.
• 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.
• Roundabouts have fewer annual injury crashes than rural two-way stop-controlled intersections, regardless of minor street demand volumes.
• 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:

• High rates of crashes involving conflicts that would tend to be resolved by a roundabout (right angle, head-on, left/through, U-turns, etc.).
• High-crash severity that could be reduced by the slower speeds associated with roundabouts.
• Site visibility problems that reduce the effectiveness of stop sign control (in this case, landscaping of the roundabout needs to be carefully considered).
• 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:

• 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?
• 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.
• 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.
• The proximity of generators of significant traffic that might have difficulty negotiating the roundabout, such as high volumes of oversized trucks.
• The proximity of traffic control devices that would require preemption, such as railroad tracks or drawbridges.
• 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.
• 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.
• Heavy pedestrian or bicycle movements in conflict with high traffic volumes pose problems for all types of traffic control.
• 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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