The Monday Morning Commute: Ask the traffic engineer: What is the difference between roundabouts and traffic circles?

By Paul Basha, traffic engineer, Summit Land Management

What is the difference between roundabouts and traffic circles?

Some traffic engineers believe there is no difference between roundabouts and traffic circles. In terms of design, there is minimal difference. Roundabouts are larger, traffic circles are smaller. The purposes between roundabouts and traffic circles are very different.

Roundabouts are a much more efficient alternative to traffic signals or four-way stops. Traffic circles are for neighborhood streets to encourage slow speeds. Neither roundabouts nor traffic circles are perfect in every situation – mimicking humans that way.

Traffic circles must gently force vehicles to deviate from straight line speeding. The road must be sufficiently wide to fit the traffic circle, then narrowed to allow drivers to proceed slowly. Often traffic circles are combined with other traffic calming devices to reinforce slow travel speeds.

And in Scottsdale, even small traffic circles can be attractive, such as near the Western Spirit Museum of the West at Marshall and Main, thanks to Ed Mell’s Jack Knife. Traffic circles are intended for low volume roads to prevent speeding.

Roundabouts are intended for intersections where there are many cars from all approaches. Roundabouts are only effective with moderate traffic volumes. If traffic volumes are too low, drivers become inattentive of the potential for other traffic. Some turn left in front of the roundabout or some drive straight across curving lane lines. If traffic volumes are too high, chaos results. Would a signal be better?

Roundabouts became common in Scottsdale early this century. (A phrase reminiscent of the recent, short-lived, historically educational, television series, “Timeless”.) The Council-adopted 2016 Transportation Master Plan states, “Roundabouts shall be the first consideration for all intersections of one or two-lane-per-direction streets.

Traffic signals should only be installed or remain if specific analysis justifies their superiority. Traffic signals, by definition, stop some cars so other cars can go. Traffic signals are so dedicated to their responsibilities that they often stop cars when there are no other cars to go. Instead of the time-consuming go-stop-go tedium of traffic signals and four-way stops; roundabouts recognize that continuously slow-moving cars travel faster than cars braking, waiting, and accelerating.

In 2005, the intersection of 96th and Sweetwater was converted from a four-way stop to a roundabout instead of a traffic signal. No more did people always have to stop whether or not a conflicting vehicle was approaching. If a conflicting vehicle approached, the drivers adjusted their speed to merge – kind of like some people do illegally and unsafely at stop signs.

Roundabouts were installed in 2008 on Cactus Road at 100th Street, 104th Street, and 108th Street. Likely all three intersections would have signals now if not for the enlightened use of roundabouts.

In 2018, the City of Scottsdale began construction of a roundabout at the intersection of 60th Street and Dove Valley Road. The adjacent Cactus Shadows High School officials, citing new drivers, much desired the known of a traffic signal rather than the unknown of a roundabout. After completion; when students, faculty, and administrators experienced driving the roundabout; they much preferred the roundabout to a traffic signal.

The first multi-lane roundabout in Scottsdale opened in 2013 at the intersection of Hayden Road and Northsight Boulevard. The long queues of stopped vehicles when the intersection was a signal did not reappear with the intersection as a roundabout. Traffic volumes increased, collisions decreased slightly, collision rates decreased, injury collisions decreased dramatically, and travel times decreased.

The second multilane roundabout in Scottsdale opened in March 2018. This roundabout serves the Mustang Library on the east and Scottsdale Fiesta on the west. It is approximately 350 feet south of the signalized main entrance to HonorHealth Shea on the east and Scottsdale Fiesta on the west. This roundabout received international acknowledgement in the November 2019 edition of the Institute of Transportation Engineers Journal Public Agency Showcase. The design was recognized for safe and efficient accommodation of pedestrian access, car travel, and five bus routes. The project also includes bicycle path connections to the east to the Camelback Walk.

The close proximity of a traffic signal and a roundabout provides a perfect opportunity to compare their operation. Include on your bucket list for this fall, standing on the sidewalk (not yet as historic as Winslow) between the two operations and decide for yourself which has less delay and less collision potential.

While there, ponder two stunning aspects of the project. The spiral staircase landscaping captures and filters rainwater directing it into the ground – saving rainwater from entering the street. Throughout the area are 20 sculptures of birds reading books. Mary Lucking’s beautiful work is named Birdie Umwelt and was included in the high international honor of 100 entries for the 2020 CODA People’s Choice Awards.

Curious about something traffic? Call or e-mail Paul at (480) 505-3931 and pbasha@summitlandmgmt.com.

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