Key Number: 12093
Project Number: A012(093)
The Idaho Transportation Department (ITD) built a new interchange in the Chubbuck/Pocatello area. The interchange connects U.S. 91/Yellowstone Highway over the Interstate 86 system. The interchange is a diverging diamond interchange (DDI).
Project Purpose and Need
The purpose of the project was to replace the existing bridge, provide increased capacity of the current interchange and increase safety. The result will be less congestion and fewer crashes. The need began when the existing bridge was determined to have reached the end of its life and needed replacement. Replacing the existing bridge created an opportunity to examine traffic and make modifications to increase capacity. With several alternatives being modeled and analyzed, the diverging diamond interchange performed the best within budget limits.
The design of the interchange started in 2010 when the concept was developed and modeling of the different alternatives were produced. Modeling and preliminary costing for a standard diamond, a wide diamond and a diverging diamond were presented to the transportation department. A single point urban interchange was initially considered, but ruled out early in the process because less expensive alternatives operated adequately for 35 years or more.
Of the three alternatives modeled, the diverging diamond interchange performed better than the other two alternatives, and was more cost effective. It allowed the construction of the interchange within the existing right of way and utilized new construction that was done in 2010 on U.S. 91 and two of the interchange ramps.
The major goal is to upgrade the bridge structure, along with increasing the capacity and safety of the interchange. Construction is anticipated to conclude in November of 2013.
- No option to purchase right of way.
- Some of the new construction on U.S. 91 will be reworked.
Why a Diverging Diamond (DDI)?
A diverging diamond interchange offers significant benefits when compared to a traditional diamond interchange. The largest benefit is 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. A traditional diamond interchange has 26 conflict points. The diverging diamond interchange has 14.
One challenge the diverging diamond interchange presents is driver education/expectancy. These types of interchanges are new in Idaho. Crossing over into opposite lanes is not conventional, and may cause some confusion to unfamiliar drivers. However, public comments after existing diverging diamond interchanges have been opened indicate the interchanges are simple and easy to navigate.
The following are links to a diverging diamond interchange in Springfield, Mo.
Click the link to see what a diverging diamond interchange looks like.
Click the link to drive the one in Springfield, Mo.
The Idaho Transportation Department values your input. Submit written comments using one of the forms below or contact Jesse Barrus for more information.
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• Download a comment form (word doc)
Jesse Barrus, Project Manager ITD District 5
Compliance with Title VI of the Civil Rights Act
The Idaho Transportation Department (ITD) is committed to compliance with Title VI of the Civil Rights Act of 1964 and all related regulations and directives. ITD assures that no person shall on the grounds of race, color, national origin, gender, age, or disability be excluded from participation in, be denied the benefits of, or be otherwise subjected to discrimination under any ITD service, program, or activity. The department also assures that every effort will be made to prevent discrimination through the impacts of its programs, policies, and activities on minority and low-income populations. In addition, the department will take reasonable steps to provide meaningful access to services for persons with Limited English Proficiency.