A modern roundabout is built with a large, often circular, raised island
located at the intersection of an arterial street with one or more crossing
roadways and may take the place of a traffic signal. Traffic maneuvers
around the circle in a counterclockwise direction, and then turns right
onto the desired street. All traffic yields to motorists in the roundabout
and left-turn movements are eliminated. Unlike a signalized intersection,
vehicles generally flow and merge through the roundabout from each approaching
street without having to stop.
Roundabouts need to accommodate pedestrians and bicyclists. It is important that automobile traffic yields to pedestrians crossing the roundabout. Splitter islands at the approaches slow vehicles and allow pedestrians to cross one direction of travel at a time. Single-lane approaches can be designed to keep speeds down to safer levels and allow pedestrians to cross. Multi-lane approaches have higher speeds, create multiple threats for pedestrians, and are not recommended.
Wayfinding and gap selection cues need to be adequately addressed in the design of roundabouts so that roundabouts are not a barrier to pedestrians with vision impairments. One possible solution is the use of accessible pedestrian signals placed on sidewalks and splitter islands to indicate both where to cross and when to cross. More research is currently underway through the National Cooperative Highway Research Program (NCHRP) to further explore the problem and develop potential solutions. Refer to NCHRP Project 3-78, Crossing Solutions at Roundabouts and Channelized Turn Lanes for Pedestrians with Vision Disabilities (at www4.trb.org/trb/crp.nsf/NCHRP+projects) for the latest status report.
Bicyclists are also disadvantaged by roundabout design. Unless the road
is narrow (one lane in each direction), speeds are slow, and traffic
very light, bicyclists may not be able to share the road comfortably.
Marking bicycle lanes through the roundabout has not been shown to be
safer. In larger roundabouts, an off-road bicycle path may be necessary
to allow cyclists to use the pedestrian route. This is inconvenient and
takes longer but it will improve safety. Refer to the FHWA report Roundabouts,
An Informational Guide for more information related to the design of facilities
for both pedestrians and bicyclists.1