PHOENIX (3TV/CBS 5) – The long-awaited diverging diamond interchange on Interstate 17 at Happy Valley Road opened Sunday. The DDI replaces roundabouts.

Here's a look at how the Diverging Diamond interchange works

This type of interchange was proposed by the University of Maryland in 2000. The first one was implemented in Springfield, Missouri nine years later. The north Phoenix DDI is one of almost 100 such interchanges in 29 states, and more are in the planning and construction phases.

"The design has increased in popularity because of safety, operational and cost benefits," explains the Arizona Department of Transportation's website. "DDIs can reduce the number and severity of crashes compared to traditional diamond interchanges while reducing traffic congestion."

DDIs are unique in that drivers briefly cross to the wrong side of the road. That means cars don't have to turn left in front of traffic.

"Traffic does cross as you're approaching the bridge on either side, but it's a matter of just getting used to the fact that that's going to happen," ADOT spokesman Doug Nintzel said Friday morning. "Be ready for the fact that it does look a little different. It does operate a little differently."

DDIs mean there are fewer traffic lights, too, which means drivers move through the interchange faster.

"Because you're never turning left across traffic, there's no need for a left turn arrow. So, traffic moves a little more smoothly with a diverging diamond," ADOT spokesman Tom Herrmann explained in August 2019, ahead of two DDIs opening on the South Mountain Freeway at 17th Avenue and Desert Foothills Parkway. These were the first DDIs in Arizona.

New type of traffic interchange in Phoenix will have you driving on the left
New South Mountain Freeway to utilize "diverging diamond interchange"

Plans for the DDI at Happy Valley Road go back to 2017.

New interchange design to replace Happy Valley roundabouts

"There was a need to reconstruct the interchange here – make it larger, give it more lanes …," Nintzel said. "Seeing what has happened with diverging diamonds around the rest of the country, this was a great candidate location," Nintzel said.

The I-17 on- and off-ramps at Happy Valley Road were closed from 5 p.m. Friday through 5 p.m. Sunday. Crews did lane striping and other work on the DDIs. Happy Valley Road also was also closed in both directions approaching I-17. 

Although the Happy Valley Road DDI is open, there is still some work to be done. That means lane restrictions will be in place. Construction is expected to wrap up later this fall.

Another DDI is planned in the Tucson area in the next few years.


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