Academic Minute Podcast

Deogratias Eustace, University of Dayton – Why Roundabouts? Known Benefits of these Circular Intersections Explained

No one likes waiting at a red light; enter the roundabout.

Deogratias Eustace, professor and director of graduate studies at the University of Dayton, looks around to find out how traffic circles can make a big difference for drivers.

Professor of transportation engineering in the Department of Civil and Environmental Engineering and Engineering Mechanics at the University of Dayton. His interest research areas include traffic safety, data analytics in transportation engineering, impact of connected vehicles technology on traffic safety and operations, and emerging mobility services. He is the department’s director of graduate studies and director of the Transportation Engineering Laboratory (TEL). He graduated with a Ph. D. in civil engineering from Kansas State University.

Why Roundabouts? Known Benefits of these Circular Intersections Explained

Roundabouts, also known as traffic circles or rotaries, are circular intersections. They offer several advantages over conventional intersections controlled by traffic signals or stop signs, but by far the most important one is safety. While a relatively new traffic control measure, roundabouts are catching on across the United States. The first modern roundabout in the U.S. was built in Las Vegas, in 1990. Over the past two decades, that number has grown to over 10,000.

Studies have shown that when a roundabout replaces a stop sign-controlled intersection, it reduces serious and fatal injury crashes by 90%, and when it replaces an intersection with a traffic light, it reduces serious and fatal injury crashes by nearly 80%. Where does this magic come from? It is simple. A roundabout reduces potential conflict points from 32 at a conventional four-way intersection to 8. The more conflict points, the more likely vehicles are to crash. At roundabouts, vehicles don’t cross each other at a right angle, and there are fewer points where vehicles merge or diverge into or away from each other.

They use yield-at-entry regulations which means vehicles drive through them more slowly with splitter islands (raised curbs where vehicles enter and exit) controlling vehicles’ speeds. These features improve traffic flow, reduce congestion and improve safety.

The roundabout’s tight circle forces approaching traffic to slow down and yield to circulating traffic, and then move smoothly around the central island. As a result, roundabouts have fewer stop-and-go issues, which reduces fuel consumption and vehicle emissions and allows drivers to perform U-turns more easily. Since traffic flows continuously at lower speeds in a roundabout, this continuous flow minimizes the need for vehicles to stop, which reduces congestion.

The current roundabout’s widespread adoption suggests that these circular intersections are here to stay.

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