Iowa Department of Transportation

GRIMES, Iowa – Sept. 21, 2020 –  The Interstate 35/First Street interchange in Ankeny will begin reopening Sept. 24.

The project will reopen in stages to allow motorists to get used to the new traffic patterns.

  • Sept. 24 – First Street and the entrance ramp to northbound I-35
  • Week of Sept. 28 – The exit ramp from northbound I-35 to First Street
  • Week of Oct. 5 – The entrance ramp from First Street to southbound I-35
  • Early November – Southbound I-35 and the southbound exit ramp from I-35 to First Street

All scheduled opening dates are weather dependent.

For the first stage of opening, traffic on First Street will see dramatic changes to the traffic patterns under the I-35 bridges, in a configuration that is known as a diverging diamond. As First Street traffic moves through the interchange, they gently crossover – or diverge – from the right side of the road to the left side of the road and then back again. This configuration significantly reduces delay because it allows all turning movements to be free-flowing, allowing most cars to enter and exit I-35 and make left turns without needing a stoplight, increasing the efficiency of the interchange.

In addition, this innovative design, when used at an interchange such as this, is safer by limiting places where vehicle paths merge or cross. A national study published in the Transportation Research Record found that a diverging diamond interchange virtually eliminates left-turn crashes and can reduce all crashes by 46 percent.

Diverging diamond interchanges are relatively new to Iowa, they have been used in other states for years. While this crossover may seem different at first, it is very intuitive for the driver as you travel through the intersection. 

This video provides an overview of driving on a diverging diamond interchange.

Work on the $63 million interchange project began in November 2017 and has been completed in stages with project completion slated with the opening of the southbound exit ramp from I-35 to First Street later this fall.

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Contact: Brian Meyer at 515-986-2863 or brian.r.meyer@iowadot.us.

Source: iowa DMV

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