Sunday, July 19, 2009

Round and round we go -- safely

A recent post on the Project for Public Spaces blog touts the virtues of the modern roundabout for both improving traffic flow through an intersection, improving safety (39% fewer crashes, 76% fewer injury crashes, and 90% fewer fatalities) and improving conditions for pedestrians and bicyclists. Improving traffic flow at in intersection has the nice collateral benefit of reducing the negative consequences of idling traffic: wasted gas, increased emissions, noise.

What improves traffic flow in a roundabout is the same thing that improves safety. Everybody goes and everybody goes slowly. At a traffic light, there is wide variance in speeds. Obviously, some cars are stopped. Others are traveling at or above the speed limit on the road. Others speed up to make it through the intersection. In the case of traffic trying to turn left, you can have a car or cars stopped and traffic speeding by in both directions.

The physical constraints of a roundabout -- center island and tight turning radii -- prevent cars from going above a design speed of 20 MPH or so. And, the absence of stop lights or signs means that nearly all cars can proceed through the intersection without stopping. When they do need to stop -- to allow a pedestrian to cross, for instance -- cars are going a speed that makes it safe and easy to do so. And, the few cars stopped at a roundabout are not stopped like cars at a light, for the entire period of a light cycle -- or multiple light cycles.

There is one other technical nicety. In a roundabout, cars never cross lanes of oncoming traffic. There is simply no opportunity for a head-to-head or t-bone crash. All potential conflict is at smaller angles -- sideswipes, not t-bones. And, at lower speeds.

Another way to think about roundabouts is that they reduce the range of behaviors in the intersection, simplifying decision-making and reducing the types of conflicts. All traffic joins from the right. The range of speeds is much narrower. No traffic crosses a lane.

Of course, in New England, we have rotaries, which are not universally viewed as safe and easy-to-use. But, rotaries, like the Horace James Circle near the Putterham Golf Course in Brookline, are not roundabouts in a very significant respect: they aren't designed to limit speeds. Horace James Circle can be navigated at very high speeds. Tighten it up (a lot) and it would be a very different beast.

Here's the Mayor of Carmel, Indiana, singing the praises of roundabouts. At 4:35, he discusses the difference between roundabouts and rotaries:


required field said...

I love the idea of roundabouts, but the jury's still out on how how they affect bicyclists and pedestrians, not limited to persons with reduced motility or impaired vision.

Anonymous said...

Great posting .. I LOVE, LOVE, LOVE 'roundabouts', aka rotaries, for all the reasons you listed. In addition to safety features, they use no electricity for lights, are completely and automatically dynamic in handling differing traffic loads, and can be structured to accomodate variously shaped road intersections.

MamaVee said...

interesting as you were writing about the speed reducing qualities I thought- Have ya ever been at any of the rotaries on Cape cod in full summer traffic? Sadly every rotary I've had the displeasure of being on in MA has been fast paced, filled with people using different rules and no where I'd want to be on foot or bike, near it. Too bad.

required field said...

To see what roundabouts would look like here, please see the Newton Centre Intersections Draft Traffic Study, Fall 2008 @

Steve Runge said...

@MamaVee: The old rotaries have big circumferences, and therefore allow high speeds. The one at Jamaicaway & Centre St. is one of my personal favorite "indy 500" rotaries. Roundabouts have such a tight circumference that few drivers exceed 20-25mph. I was skeptical, too, but I'm sold.