Idaho’s first diverging diamond interchange receives award

Boise, Idaho — The Chubbuck Diverging Diamond Interchange, which connects U.S. 91/Yellowstone Highway over the Interstate 86 system in Chubbuck, Idaho, has received the Idaho Transportation Department’s (ITD) 2014 Excellence in Transportation Award in the large design category. The Excellence in Transportation Awards are sponsored by the ITD to recognize outstanding initiatives in developing, planning, and implementing transportation projects throughout the state.

The project replaced an existing bridge, provided increased capacity of the current interchange, and increased safety. Parsons Brinckerhoff worked closely with ITD District 5 to lay the groundwork for this innovative freeway interchange solution. The firm provided preliminary engineering, which was then used to evaluate interchange types against a set of criteria that included cost, performance, right-of-way impacts, access impacts to existing businesses, and other criteria.

The evaluation led to a recommendation to construct a diverging diamond interchange, which offers benefits when compared to a traditional diamond interchange, the most significant being continuous left-turning traffic. Because traffic is crossed over to the opposite side of the bridge, there is no waiting for left turns onto the interstate. This eliminates a phase for the signals and eliminates the potential for crashes with left-turning traffic.

Once ITD received permission to move the project forward, Parsons Brinckerhoff prepared an interchange access modification request for the Federal Highway Administration, as well as a noise study for the project area. ITD completed final design for the project, and construction was completed in 2013 on the state’s first diverging diamond interchange.  


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