From Musings to Spreading Concept: The Diverging Diamond Interchange Moves Across PA

​What started out as some musings by a University of Maryland engineering graduate student in 2000 has swept the country and is now spreading across Pennsylvania. The transportation innovation is the Diverging Diamond Interchange (DDI) and is among the Federal Highway Administration's Every Day Counts innovations championed by the Pennsylvania State Transportation Innovation Council (STIC).

PennDOT's District 12 opened the first DDI in Pennsylvania in September 2016 at the Interstate 70/ U.S. 19 interchange in Washington County. The second is now under construction in PennDOT's District 8 at the Shrewsbury interchange of Interstate 83 in York County, and a third is planned for the U.S. 222/U.S. 322 interchange in Lancaster County. District 12 is also designing its second DDI at the Interstate 70/State Route 51 interchange in Westmoreland County.

Graduate student Gilbert Chlewicki, who had been drawing sketches of highways since he was in elementary school, had to do a term paper and wanted to come up with an innovative design as the subject. He was inspired by an existing interchange at Interstates 95 and 695 north of Baltimore and, using that as a starting point, developed the design for what became the DDI. The first one was installed in Springfield, Mo., in 2009, and the I-70/U.S. 19 was the seventy-third to open. There are now 98 across the country.

Safety is the big benefit of the DDI design.

"Compared to a conventional diamond interchange, the DDI reduces vehicle-to-vehicle conflict points by nearly 50 percent and eliminates many of the most severe crash types," according to FHWA.

The key is the elimination of left turns across oncoming traffic. At the interchange, traffic in both directions is transitioned or crossed over to the left side at a signal-controlled point and then transitioned back to the right side at a second signal-controlled point. At the second point, left turns can be made without crossing oncoming traffic.

"As these have become more popular, we're seeing results that are good across the country," Governor Tom Wolf told the media at the ceremonial ribbon cutting for the I-70/U.S. 19 DDI.

Added then PennDOT Secretary Leslie S. Richards: "It is improving safety by reducing traffic contact points."

Driver reaction was positive as well.

"At first, it did (confuse me) because you think you're on the wrong side of the road when you're going to the left, but now it's fine," Janie Gilbert of Claysville told WTAE-TV. "It makes the traffic flow quicker."

District 12 Executive Joe Szczur said the project was a long-standing priority. Besides the added safety, the DDI required less space and reduced property takes than a traditional interchange, he noted.

PennDOT took special care as the design moved forward, involving its maintenance staff to ensure the design could accommodate snow removal, closely following Missouri's "DDI Lessons Learned" document, and developing a video to take to public meetings to gain driver acceptance.

District 12 Project Manager and Designer Barry Lyons noted that a brochure and animated video the district deployed played a key role in building public support.

"The brochure and video were displayed at public meetings and placed on the I-70 project website," he said.

Lyons added that the district incorporated peer reviews in its planning and, in fact, engaged Mr. Chelwicki in the process.

"As far as buy-in and support for innovations, the key is communication," said Rachel Duda, District 12 assistant district executive for design.

She added that the district engaged through public meetings, private clubs and organizations, conferences, and news media, created brochures for both DDI and roundabouts and offered driving simulations on its website.

"The driving simulations … helped immensely," she said.  "People understood the project better when they were watching how easily it could be done."

Added District 8 Executive Mike Keiser:

"The DDI concept not only provides for improved capacity, now and in future years, but also improves safety by eliminating several points of conflict compared to traditional signalized intersections. We are also appreciative for the support from our Metropolitan Planning Organization (MPO) partners and local municipalities, including emergency responders, regarding the selection of the DDI alternative.  We are looking forward to the successful completion of the [Shrewsbury interchange] project so that area motorists can begin to experience all the benefits associated with the DDI."    

Moving forward, PennDOT will continue to look for ways to advance the DDI concept across Pennsylvania.