Opening soon: The new diverging diamond interchange on I-17

By Laurie Merrill / ADOT Communications

It’s happening!

The new diverging diamond interchange (DDI) at Interstate 17 and Happy Valley Road is scheduled to open to motorists sometime in mid-October.

The innovative Arizona Department of Transportation interchange paves the way for a slew of benefits: Better traffic flow, fewer “conflict points” between traffic traveling in opposite directions and thus, improved safety.

ADOT has worked closely with the city of Phoenix and Maricopa Association of Governments (the Valley’s freeway planning agency) in designing and now building the state’s first major diverging diamond interchange. The DDI design has been growing in use across the country for its innovative features. 

As mentioned above, diverging diamonds provide safety benefits by reducing the number of conflict points at intersections within the interchange. Those are locations with opposing directions of traffic. A potential conflict exists every time a vehicle crosses or turns across the path of another direction of traffic.

For example, at the Happy Valley Road DDI, intersections and traffic signals will allow drivers to cross to the left side of the bridge over I-17 and thus have a direct turn onto the freeway on-ramp. They are able to make that left turn without facing traffic in the opposite direction.

Reducing such conflict points as well as overall traffic-signal movements also allows more traffic to move through the interchange in a shorter amount of time. That helps to limit wait times for drivers, especially during times of busier traffic.

While you may be unfamiliar with such an interchange, studies and observations in other states have shown motorists quickly get the hang of traveling in a DDI. The use of noticeable pavement markings also helps direct traffic. You can see how it all works in the video to the right..  

ADOT State Engineer Dallas Hammitt discussed the benefits of a DDI in a blog earlier this year about two smaller diverging diamonds (called “half DDIs”) now in use along the Loop 202 South Mountain Freeway.

“This type of interchange works by temporarily transitioning traffic to the left side of the road, allowing through-traffic and left-turning traffic to proceed through the interchange simultaneously, eliminating the need for a signalized left turn.”

Crash rates improved dramatically after a diverging diamond interchange was constructed in Springfield, Missouri, according to the Federal Highway Administration. The study compared crashes from the first year after construction to the five-year average before, and found the following:

  • Left-turn crashes were totally eliminated.
  • Right-angle crashes were reduced 72 percent.
  • Rear-end crashes were reduced 29 percent.
  • Total crashes were reduced 46 percent.

“Where they have been built,” the FHWA report said, “travelers save time, agencies saved money, and communities will benefit from safer facilities for many years.”