PROGRAMS AND SERVICES
TRAFFIC CALMING PROGRAM
Level 3 Traffic Calming: ROUNDABOUTS

A roundabout is a landscaped circular island located in the center of a residential intersection. Traffic drives to the right of the roundabout in a counterclockwise direction for all movements (left-turn, straight, and right-turn) Right-of-way assignment with a roundabout is given to those within the circle whereas those entering the roundabout must yield.

This type of measure introduces two traffic calming concepts: physical constraints and visual traffic calming. Level 3 traffic calming involves physical constraints which produces a point vehicle speed reduction. Point speed reductions may occur at the roundabout. But motorists speed beyond the roundabout are typically either unaffected or higher than before unless applied as a system of devices. Visual traffic calming involves installing features which provide visual breaks along long sections of roadway to make the roadway appear either shorter or as a not a through route. Roundabouts may also serve as gateways (see Level 1) to a community or to a system of traffic calming measures.

Roundabouts function somewhat similarly to bulb-outs except that vehicles must deviate from their path to drive past this traffic calming measure, thus impacting motorists speed more than bulb-outs. Visual traffic calming effects are greater with roundabouts than with bulb-outs. However, pedestrian accessibility with roundabouts is much more difficult and addressing vehicle access (turning radii) becomes more of an issue. Advance signing and pavement markings are usually installed for roundabouts to minimize collision potential.

IMPLEMENTATION

Roundabouts are beneficial on long, straight, and flat roadway sections where visual traffic calming effects or gateway features may apply. This traffic calming measure is not recommended along major collector roadways, at intersections with significant pedestrian activity, nor at intersections with levels-of-service C or worse due to the potential for motorists confusion. (level of service (LOS) is a measure of efficiency from a user perspective (LOS A being very efficient through LOS F, which is gridlock)). Design considerations include accommodating larger vehicles (particularly fire trucks), street lighting, landscaping, and the associated maintenance and sight distance issues.