Diverging Diamond Interchange Puts Drivers on the Left Side of the Road

The unusual design improves traffic flow and reduces the danger of head-on collisions as cars enter the I-5 freeway.

2 minute read

January 5, 2021, 11:00 AM PST

By Diana Ionescu @aworkoffiction


Diverging diamond interchange in Springfield, Missouri.

By directing cars to the left side of the road, the diverging diamond design eliminates the need to stop at a traffic light before turning left onto the freeway. | Michael Anderson, Bernard Schroer, Dietmar Moeller / Diverging diamond interchange in Springfield, Missouri.

If you've ever wondered what it's like to drive on the left side of the road, you can now experience it in Washington State. The "diverging diamond" interchange in Laceyopened in August of 2020, with final touches scheduled to be completed early this year. The design, writes Mike Lindblom in the Seattle Times, reduces possible collision points by nearly half and speeds up the time it takes drivers to reach the I-5 freeway. Putting cars on the left side eliminates the need for a traffic light as drivers turn left to enter the freeway and eases the flow of traffic. According to Blake Knoblauch, executive director of the Lacey South Sound Chamber of Commerce, "drivers who previously waited through two or three light cycles now usually make it through on the first green light."

Popularized by Gilbert Chlewicki of Advanced Traffic Solutions, the diverging diamond interchangeis slowly picking up steam in the United States, with around 150 constructed to date. A study of the design that analyzed 26 such interchanges found that the number of collisions declined by 37%, with the number of injury crashes falling by more than 50%.

The interchange takes a bit of getting used to for some drivers, who fear getting hit or have trouble reaching the appropriate lane for entering the freeway. Bicycling conditions "remain hostile," says Lindblom, an issue acknowledged by Chlewicki. The project is primarily a measure to accommodate projected traffic growth through 2025 as local authorities plan for more expansions to facilitate truck traffic to local warehouses.

Monday, December 28, 2020 in Seattle Times

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