Why Engineers are Talking about Roundabouts

Posted by on Sep 17, 2013

Roundabouts may not be prevalent enough in the Southeast that you would see one every day, but they are quickly gaining popularity in the region.

At our Sain Associates office in Mandeville, Louisiana, I have worked on roundabouts at several locations that were suggested for a variety of reasons. Some are being studied to replace existing traffic signals, while others are planned for intersections within shopping centers. Some are also being studied because of crash history or excessive delay.. It’s now required that the roundabout be evaluated at any intersection where a traffic signal is recommended.

They are a great option to consider because roundabouts lend themselves to lower speeds, and therefore crashes tend to be less severe. The opportunity for accidents also tends to be reduced. The number of places or opportunities where vehicles could crash in an intersection is called a conflict point. In a traditional four leg intersection, there are 32 conflict points.  Half of those are considered crossing conflicts, which are the movements that tend to result in more severe crashes for both injuries and damage to vehicles.

In a single lane roundabout, only eight conflict points exist, and none of them are crossing conflicts. Instead they are merge and diverge conflicts, which tend to be more rear end conflicts instead of the “right angle” type that are often seen in the traditional intersection design.  The conflict types seen with roundabouts tend to produce less severe crashes than those experienced at traditional intersections.

Another reason more developers and designers are incorporating roundabouts is that they are more attractive than traffic signals since the center island that traffic flows around can be landscaped.

We are seeing a lot of activity related to roundabout installations in Louisiana. The State currently has approximately 18 modern roundabouts constructed with another 40 to 50 at various stages of study, planning, and construction.

The trend is moving throughout the Southeast.  Georgia DOT has had a roundabout policy in place for a few years.  Mississippi DOT is beginning to encourage the use of roundabouts as a safety improvement tool, and Alabama DOT is currently developing a roundabout design guide for use statewide to encourage uniformity in design and greater use of roundabouts.

Sain Associates, Inc., is headquartered in Birmingham, Alabama, with offices in Cullman, Alabama, Pulaski, Tennessee and Mandeville, Louisiana. Sain is a site engineering, traffic/transportation engineering and planning and land surveying firm with experience in more than thirty states. 

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