Cycle facilities at roundabouts: Encyclopedia - Cycle facilities at roundabouts

Cycle facilities at roundabouts

This photo shows a roundabout with cycle lanes around the outside. The particular example shown is in a country where traffic keeps to the left, and traffic in the circular roadway of the roundabout travels clockwise. In other countries, the roundabout would be essentially a mirror image of the one shown.

With cycle lanes, bicyclists do not merge into the flow of motor traffic in the roundabout, but rather, they travel around the outside, relieving them of the requirement to merge. The colored pavement and edge lines of the cycle lanes indicate that motorists are required to yield to bicyclists at all locations where their paths may cross.

The common assumption, unsupported by research, is that such an installation increases safety for bicyclists, some of whom will be children or novices who lack the skills to execute a merge. Another underlying assumption is that mixing with motor traffic is inherently dangerous for any bicyclists, because motor vehicles are fast, large and massive.

Research, however, has shown that even in relatively large circular intersections that lack modern roundabout design features, a high rate of bicycle/motor vehicle crashes occurs when bicyclists are riding around the outside. A 1992 study from the German Transport Ministry's research institute (Ref. 1) has cast particular light on this issue. The study found that bicyclists' risk is relatively high in all such intersections, but it is much higher when the intersection has a marked bicycle lane or sidepath around its outside. The results of this study concerning circular intersections are summarized on a web site (in German, but partially translated below) (Ref. 2).

Collisions typically occur when a motorist is entering or leaving the circular roadway. A motorist entering the circular roadway must yield to traffic in it, but such traffic will generally keep away from the outside of the circular roadway (as with a vehicle in the photo) if passing an entrance. A bicyclist close to the edge of the roadway is not in the usual position where an entering motorist expects to look for circulating traffic.

When exiting the circular roadway, a motorist must look ahead to steer, and to avoid colliding with another vehicle ahead or with pedestrians in a crosswalk. As the circular roadway curves away from the exit, the path of a vehicle exiting the circular roadway is relatively straight, and so the exiting motorist may often not need to slow substantially. However, if it is necessary to yield to a bicyclist riding around the outside, the exiting motorist must look toward the rear, to the outside of the intersection. With many vehicles, such as vans, the driver's view in this direction is obstructed. The task burden of the motorist is therefore substantially increased if bicyclists ride around the outside. The resulting conflicts, and more frequent requirements for motorists to slow or stop, also reduce the efficiency of traffic flow which is one of the major advantages of the circular intersections. Cycle lanes around the outside of circular intersections are therefore falling out of favor.

The special features of modern roundabouts, including splitter/diverter islands and relatively small diameter of the circular roadway, decrease the speed of motor traffic and so reduce the risk of collisions for bicyclists as well as motorists below that of conventional intersections. Design guidance for modern roundabouts (for example Ref. 3) recommends terminating cycle lanes (if any) well before the entrances, so bicyclists merge into the stream of motor traffic. Bicyclists who lack the skill and confidence to do this may use the crosswalks as pedestrians. Modern design guidance recommends placing the crosswalks far enough from the roundabout so that at least one exiting vehicle can wait without blocking the circular roadway.

Adapted from the Wikipedia article "Cycle facilities at roundabouts", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki