Driving Safety Home: DDIs designed to improve traffic flow, safety

Driving Safety Home: DDIs designed to improve traffic flow, safety

By Dallas Hammit / ADOT State Engineer
January 31, 2020

Besides being the first highway project in Arizona to be built using a public-private partnership, the Loop 202 South Mountain Freeway is also ADOT’s first freeway to use half-diverging diamond interchanges.

The new section of freeway has two of them – one at Desert Foothills Parkway and another at 17th Avenue.

A diverging diamond interchange (DDI) helps with traffic flow and safety. 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.

It’s a much easier concept to understand once you see it, so I suggest you check out the ADOT video that simulates driving through the South Mountain Freeway half-diverging diamonds. You can find that video, along with more details at

You might be wondering what the difference is between a half-diverging diamond interchange and a full diverging diamond interchange.

A half DDI is used when one travel direction does not extend past the interchange, limiting access to only three directions. Because Desert Foothills Parkway and 17th Avenue don’t extend south of the freeway, there was no need for a full DDI. If a future connection to the south is needed, it can easily be implemented with the current design.

When you drive it, I hope you notice the innovation and dedication it took to construct the Loop 202 South Mountain Freeway. I also hope you’ll go the speed limit, avoid distractions and wear your seat belt.