George Williams admits he was pretty lukewarm about roundabouts when he started working for the city of Scottsdale.
“I thought of them as a small tool in the toolbox,” said Williams, Scottsdale’s principal traffic engineer. “They had their place here and there.”
But after encountering impressive national and international data on roundabout safety at conferences and workshops, Williams began to pay them more attention.
“You just can’t ignore a 90 percent reduction in fatalities and serious injuries,” said Williams. “Nothing else in the toolbox was working that well.”
A recent study by Arizona State University that looked at 17 Arizona intersections converted to roundabouts backed up the safety data. Overall injury severity was reduced with both single-lane and double-lane roundabout conversions.
“Anytime you’re considering a signal, you should be considering a roundabout,” said Williams. “The roundabout will win out more times than not.”
The reason roundabouts are able to so effectively reduce injury severity is that all traffic moves in the same direction around a raised center island, eliminating the possibility of “T-bone” and head-on collisions, which usually result in the worst injuries.
Modern roundabouts also keep all traffic to a steady 15 to 25 miles per hour and require all entering vehicles to yield to those already in the circle. Roundabouts also keep pedestrian crossings outside of circle. All of these elements help reduce the likelihood and severity of crashes.
Of the six double-lane roundabouts that the ASU study looked at, Scottsdale’s roundabout at the intersection of Hayden Road and Northsight Boulevard had the most impressive results. In addition to reduced injury severity, the roundabout also had fewer collisions and injuries overall. The annual injury rate was reduced from 5 injuries per year when the intersection was signal controlled to 0.5 injuries year with a roundabout — a 90 percent reduction.
Williams attributes the Hayden/Northsight roundabout’s success to the city’s meticulous attention to design.
“There are six major geometric parameters and dozens of minor design elements that can be tweaked to impact safety, capacity, speed control and access,” said Williams. “There isn’t one cookie cutter design that works better than another. It depends on the individual roundabout.”
Choices in road curvature, entry angles, landscaping, pedestrian crossings, lane markings and signage all impact how well a roundabout works. Design must also contend with site constraints, such as budget, parking needs, buildings and available open space and right of way. Scottsdale’s Transportation Department consulted with a specialist (who had designed more than 300 roundabouts) to help take all of these competing factors into account at the Hayden/Northsight location.
Modern roundabouts not only improve safety, said Scottsdale Transportation Director Paul Basha. They’re more efficient, better for the environment and easier on vehicles.
Despite all the advantages, roundabouts are not the best choice for every intersection. Basha said that roundabouts work best with specific traffic volumes and patterns.
“If traffic volumes are too low, people will attempt to drive too fast,” said Basha. “And if traffic volumes at times are extremely high, then a traffic signal is possibly a better choice.”
If there is a very large difference between traffic volumes on the intersecting streets, a stop sign or vehicle-triggered light is sometimes more appropriate.
Finally, intersections that include roads with three or more through lanes would likely confuse and frustrate drivers, as roundabouts are still a relatively new traffic control device in the Western U.S.
But Basha believes over time we’ll likely see fewer signalized intersections as more research backs the benefits of roundabouts and as drivers become more familiar and comfortable with them.
“In reality, traffic signals are primitive,” said Basha. “Some cars go and some cars stop, which is inefficient and not as safe.”
A new multilane roundabout is under construction on 90th Street at Mustang Library and is expected to be complete by the end of the year. The busy area also includes HonorHealth Scottsdale Shea Medical Center and several shopping centers. A roundabout was determined to be the best option to reduce left-turn collisions and provide a safer option for U-turns and pedestrian crossings.
The city also plans to construct roundabouts on Raintree Boulevard at the intersections of 73rd Street, 76th Street and Hayden Road to improve capacity, safety and traffic flow as this corridor has a high turn volume. Construction is expected to begin in 2018.
For more information on roundabouts in Scottsdale, visit ScottsdaleAZ.gov and search “roundabouts”
