Friday, February 8, 2013



During a meeting with a Burlington (VT) city official this week, I was asked to give a concise reasoning of how roundabouts increase safety for walkers.  In response quickly went into the studies, statistics, etc., but forgot entirely the basic element which reduces the frequency and severity of walker injuries: roundabouts by design reduce the speeds of vehicles.  Only by lowering speeds of vehicles can one begin to address walker safety—and the only way to reduce vehicle speeds is with concrete and steel impediments which roundabouts and other traffic calming devices provide.

The roundabout reduces speeds by forcing an approaching vehicle to divert from a straight line through placing a splitter island at the approach point with a curb as the enforcer.  After entry, the vehicle continues to face constraint by having to travel on a circular curbed travelway—the smaller the overall roundabout, the lower the circulating speed.  The entry and curved circular travelway constraints on speed are called “deflection.”  The one factor in all walker safety research which most impacts on walker injury frequency and injury severity is speed.  The small mini-roundabout (like the first one in Vermont built in Manchester Center last fall) features a humped area in the place of a curbed central island—but a mini generally can only be located at an intersection which has a fairly low speed environment.  And a mini only gets employed where a roundabout with a curbed central island is not practical.

Of course other features of a roundabout aid in the overall walker injury reduction—the splitter island providing a mid-crossing refuge so the walker deals with traffic from one direction at a time, the location of the crosswalk a car length from the actual intersection so there are not left hand turning traffic issues, and crosswalk width relatively narrow both reducing walker exposure and assisting in speed reduction of vehicles.

So with roundabouts you do get up to a 90% reduction in injury rates for walkers, and the U.S. and Canadian roundabouts record so far (no fatalities to date) approximates that of France where they yearly average about one walker fatality per 15,000 of their roundabouts yearly (well over 30,000 there today).

Hope this gets the forest and trees back into proper perspective.

1 comment:

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