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

portrait of professional woman

I love the variety of courses, many practical, and all richly illustrated. They have inspired many ideas that I've applied in practice, and in my own teaching. Mary G., Urban Planner

I love the variety of courses, many practical, and all richly illustrated. They have inspired many ideas that I've applied in practice, and in my own teaching.

Mary G., Urban Planner

Cover CM Credits, Earn Certificates, Push Your Career Forward

Aerial view of town of Wailuku in Maui, Hawaii with mountains in background against cloudy sunset sky.

Maui's Vacation Rental Debate Turns Ugly

Verbal attacks, misinformation campaigns and fistfights plague a high-stakes debate to convert thousands of vacation rentals into long-term housing.

July 1, 2025 - Honolulu Civil Beat

Logo for Planetizen Federal Action Tracker with black and white image of U.S. Capitol with water ripple overlay.

Planetizen Federal Action Tracker

A weekly monitor of how Trump’s orders and actions are impacting planners and planning in America.

July 9, 2025 - Diana Ionescu

Map of Haussmann's redesign of Paris in the 1850s through 1870s under Napoleon III.

In Urban Planning, AI Prompting Could be the New Design Thinking

Creativity has long been key to great urban design. What if we see AI as our new creative partner?

June 30, 2025 - Tom Sanchez

View of dense apartment buildings on Seattle waterfront with high-rise buildings in background.

King County Supportive Housing Program Offers Hope for Unhoused Residents

The county is taking a ‘Housing First’ approach that prioritizes getting people into housing, then offering wraparound supportive services.

July 11 - Real Change

Aerial view of suburban housing near Las Vegas, Nevada.

Researchers Use AI to Get Clearer Picture of US Housing

Analysts are using artificial intelligence to supercharge their research by allowing them to comb through data faster. Though these AI tools can be error prone, they save time and housing researchers are optimistic about the future.

July 11 - Shelterforce Magazine

Green bike share bikes parked in a row on a commercial street with outdoor dining and greenery.

Making Shared Micromobility More Inclusive

Cities and shared mobility system operators can do more to include people with disabilities in planning and operations, per a new report.

July 11 - Cities Today