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. (Photo: Hayden/Northsight Roundabout)
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. (Photo: Hayden/Northsight intersection before and after roundabout)
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"
Roundabout Driving Tips
- As you approach a roundabout, prepare to reduce your speed to 15 to 25 miles per hour. Road design and signage will encourage you and those around you to drive at a slow, uniform speed.
- As you approach a roundabout, slow down, look left and expect to stop. If there is no traffic in the roundabout or there is a wide enough gap between cars, however, a stop is not required. Most important, remember that entering traffic must yield to vehicles already in the roundabout.
- As you approach a double-lane roundabout, look for lane markings in the road, and choose the appropriate lane depending on where you want to go. Just like at a signalized intersection, don't expect to be able to turn left from a right turn lane.
- Don't change lanes in a multilane roundabout. If you need to exit and you find yourself in the inside lane, just go around the roundabout again. It will only add a few seconds to your trip.
- For more driving tips and an instructional video, go to ScottsdaleAZ.gov and search "roundabouts"
Debunking stubborn roundabout myths
Despite impressive safety numbers, shorter travel time and other benefits, roundabouts have been relatively slow to catch on. Scottsdale transportation officials believe this may be due to several myths surrounding roundabouts:
Myth: "Roundabouts," "traffic circles" and "rotaries" are all essentially the same thing.
"When people hear 'roundabout,' they sometimes think of the large, high-speed circular intersections more common in Europe and the eastern U.S.," said Scottsdale Principal Traffic Engineer George Williams. "Those are rotaries, not modern roundabouts."
Modern roundabouts must meet a minimum of three criteria:
- All traffic must move counterclockwise around a raised, circular center median.
With rotaries, traffic is sometimes routed through the center or under the circle.
- The roundabout must be designed to keep traffic speeds between 15 and 25 miles per hour.
Traffic speeds in the larger rotaries tend to be much faster, and neighborhood traffic circles meant for traffic calming often force very slow traffic speeds.
- All entering traffic must yield to traffic already in the roundabout.
With rotaries, multiple types of entries are often used for one rotary, including stop-controlled, merge-controlled and yield-controlled. Traffic signals sometimes control both entering traffic and traffic already in the rotary.
Myth: Roundabouts have less capacity than other intersections.
Capacity at roundabouts is typically much greater than at signals, and the capacity of an all-way stop is half at best.
Myth: The primary purpose of roundabouts is to calm or slow traffic.
While roundabouts encourage cars to travel at speeds of 15 to 25 miles per hour, the purposes of roundabouts are to improve safety, traffic flow and capacity at an intersection.
Myth: Roundabouts increase drive times.
Because of their continuous flow, travel is faster with roundabouts.
"With roundabouts, all cars move slowly," said Basha. "The go-stop-go of traffic signals and stop signs is much more time consuming."
Myth: Roundabouts cost more to construct than signalized intersections.
While it costs more to construct a roundabout than a four-way stop, it costs about the same to construct a roundabout as it does a signalized intersection. Roundabouts tend to be cheaper over time as they do not require electricity or maintenance of signals.
Myth: Roundabouts require more space than signalized intersections.
While roundabouts require more space than a four-way stop, they take up about the same or often less space than a signalized intersection, particularly when left- or right-turn lanes are necessary.