ADOT makes case for 'diverging diamond interchanges'

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(Source: Arizona Department of Transportation) (Source: Arizona Department of Transportation)
Roundabouts like one at Interstate 17 and Happy Valley Road were supposed to be a temporary fix, in place only until the state had enough money to overhaul the intersection. (Source: 3TV/CBS 5) Roundabouts like one at Interstate 17 and Happy Valley Road were supposed to be a temporary fix, in place only until the state had enough money to overhaul the intersection. (Source: 3TV/CBS 5)
PHOENIX (3TV/CBS 5) -

The Arizona Department of Transportation made its case Tuesday for a new type of freeway interchange designed to improve traffic flow and safety.

At a crowded public meeting attended by more than 200 people, ADOT outlined plans for a "diverging diamond interchange" planned for I-17 and Happy Valley Road. The agency is considering two other diverging diamond interchanges as part of the Loop 202 freeway expansion project.

[RELATED: Diverging diamond interchange proposed for I-17 at Happy Valley Road]

"We think people will find as they look at the diverging diamond, they'll see it as a safer way to move a lot more traffic through an interchange," said ADOT spokesman Doug Nintzel.

Drivers have had mixed reactions to the design since it was unveiled earlier this year. In a diverging diamond, some lanes cross in front of oncoming traffic while passing over a freeway. A website called DivergingDiamond.com explains the design and its history in detail. 

[RELATED: New interchange design to replace Happy Valley roundabouts]

The crisscross design allows for fewer stoplights.

At least 29 states have built diverging diamond interchanges since the first one was introduced in the U.S. in 2009. A study in the state of Missouri found the number of serious crashes at these interchanges dropped by 55 percent, according to Nintzel.

At the meeting, members of the public asked questions about the safety of cyclists and pedestrians in the interchanges, and the potential for wrong-way drivers.

[MORE: New South Mountain Freeway to utilize diverging diamond interchange]

Nintzel argued that the interchanges will greatly reduce the risk of a wrong-way driver entering the freeway. The lanes with the greatest potential for a wrong-way driver are the criss-crossed pass-through lanes that do not have access to the freeway. Nintzel said roadway angles, barriers, signs and markings will make it difficult for impaired drivers to head the wrong direction on those lanes.

[SPECIAL SECTION: Wrong-way drivers in Arizona]

[MAP: Wrong-way driving incidents that ended with a crash or an arrest]

The interchanges would replace the roundabouts at Interstate 17 and Happy Valley Road.

“In examining options, ADOT determined that a diverging diamond interchange would be better able to manage the growing volume of traffic at Happy Valley Road and reduce the amount of time drivers spend waiting at traffic signals,” according to an ADOT news release. “It also enhances safety by reducing the number of points where directions of travel conflict.”

ADOT posted a video of a diverging diamond interchange in Illinois to show Arizona drivers exactly how it works.

[RELATED: Diverging diamond interchange opens in Atlanta]


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