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SR 9 - SR 531 / 172nd St NE - Roundabout

October 2009

Roundabout selected as preferred option

Engineers believe that a roundabout is the best solution to improve safety and relieve congestion at the State Route 9/State Route 531 intersection in Arlington.

After an in-depth analysis of the two options – an improved signal or a roundabout – engineers chose the roundabout because it’s the best solution for this intersection.

Draft design visual of a roundabout at the SR 9/SR 531 intersection. South is towards the top left corner of the picture. SR 9 runs from top left to bottom right. SR 531 runs from bottom left to top right. 

Reducing serious collisions

The roundabout will help prevent/reduce serious collisions for the 16,000 drivers that use the intersection daily. From 2004 to 2008, there were 28 collisions that resulted in 29 injuries. The majority of those collisions and injuries occurred when drivers tried to turn left through the intersection and they were hit by (or they hit) other vehicles.

Roundabouts eliminate the worst kinds of collisions - T-bone, head-on and crossover - because all vehicles travel counterclockwise around a central island. There are also no red lights to run.

Roundabouts reduced crashes by 75 percent at intersections where stop signs or signals were previously used for traffic control, according to a study by the Insurance Institute for Highway Safety (IIHS). Studies by the IIHS and Federal Highway Administration have shown that roundabouts typically achieve:

  • A 37 percent reduction in overall collisions
  • A 75 percent reduction in injury collisions
  • A 90 percent reduction in fatality collisions
  • A 40 percent reduction in pedestrian collisions

As more development occurs near the intersection, a roundabout will continue to work effectively for the next 20 to 30 years. A traffic signal would only be effective for the next decade before drivers would begin to notice increased congestion and delays. Having an intersection that will work well for the next several decades is especially important because the SR 9/ SR 531 intersection is planned as a significant commercial and industrial center, which will contribute to increased growth in the future.


The roundabout will provide the greatest return on our investment. Our computer modeling shows, with future growth factored in, that the roundabout will continue to work efficiently 20 years from now without any upgrades or added lanes. We design and build all our projects to meet a 20-year lifespan.


For more information about everything roundabouts, visit our roundabout Web page. Here you'll find information about: How to drive a roundabout; what a roundabout is; how they work for pedestrians and bicyclists; their benefits; public opinion of them; and where they're located in the state. 

Comments and contacts

Send us your questions or comments about the project, the roundabout, or roundabouts in general with our online comment form. Your comments will be sent directly to Dustin Terpening, with the communication team, and Jason Koreski, with the design team.