Tuesday, February 2, 2010

What is a modern roundabout?

Contributed by: Sandy Paquette, Trainer, Evergreen Safety Council

Not all circular intersections are roundabouts...

Rotary: high speeds on approaches and usually within the circle, not pedestrian friendly.

Traffic circle: often signalized and/or bi-directional, sometimes nothing more than a circular roadway with numerous “T” intersections.

Neighborhood circle: traffic calming countermeasure, usually to traffic control, location would not likely warrant a traffic signal, might be in lieu of a 2-way or 4-way stop.

What is a Modern Roundabout?
There are four points that differentiate a modern roundabout from similar or related traffic control features.
A compact one-way, circular intersection in which traffic flows counterclockwise around a center island (less left turn conflicts).
Other styles (i.e. rotaries, traffic circles) may have 2-directional flow and are typically much larger than the modern roundabout. The compactness helps keep speeds low and makes it easier for the driver to stay oriented.
  • Entering traffic yields (this is true no matter how many lanes are on the approach).
  • Other styles may have circulating traffic yield to entering traffic.
  • Approaches are channelized to deflect traffic into the flow.
  • Other styles may not use channelization, or deflection may not exist.
  • Designed to slow the speed of vehicles through deflection of the vehicle path.
  • Other styles may not be designed to slow traffic.

Why a roundabout?
Improve safety; the roundabout has zero vehicle crossing conflict points as opposed to the 16 vehicle crossing conflict points at the 4-leg intersection. Pedestrians are faced with simpler decisions at a time and they travel shorter distance and with two less hazardous conflicts then at conventional intersections (whether stopped or signal controlled).

Reduce congestion and pollution, simply less time for stopped and idling vehicles.

Save money, less signaling equipment to install, less power, and less maintenance.

Complement community values, roundabouts can operate quieter than conventional signalized intersections because of fewer start-ups and braking by vehicles.
Roundabouts offer the opportunity to provide attractive entries or centerpieces to communities. However, hard objects in the central island directly facing entries are a safety hazard. The central island offers opportunities for aesthetic landscaping and pavement textures can be varied on the aprons.

For more detailed information, go to the Federal Highway Administration.

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