Engineering professor to present breakthrough traffic safety research in nation's capital

Diverging diamond a gem for traffic safety

It’s common for drivers to feel anxious, frustrated and even scared when they need to cross a bridge and make a left turn across traffic to reach a freeway entrance ramp — and they’re right to have those feelings. Common design practices for so-called “diamonds,” the paired exit and entrance ramps that lead off of and onto freeways, haven’t changed much since the 1930s creation of the German Autobahn and American parkways, the predecessors of today’s high-speed freeways. Many traditional diamonds are a recipe for mistakes that lead to crashes, and left turns in particular are extremely dangerous, says Joseph Hummer, chair of the Department of Civil and Environmental Engineering at Wayne State University.

Hummer will present major findings on the improved safety and effectiveness of a relatively recent design, the diverging diamond, at the annual 12,000-attendee meeting of the Transportation Research Board in Washington, D.C., in January. The new design separates traffic in a way that makes it safer for left-turning vehicles to reach a freeway on-ramp. Michigan just opened its first diverging diamond at I-75 and University Drive in Auburn Hills. Hummer’s team studied seven of the earliest diverging diamonds built in the U.S. in Missouri, Kentucky, Tennessee and New York. The findings include a 33 percent reduction in collisions and a 40 percent reduction in traffic injuries on average compared to a standard diamond.

“I’ve told my traffic safety classes for years if people stop making left turns tomorrow, we’d be out of business the next day. My advice for everybody is avoid being a left-turn driver,” says Hummer, with only a hint of sarcasm.

Left turns are recognized as a major source of traffic conflict and cause so much financial loss that large delivery corporations, including United Parcel Service, spend millions annually to design routes for their drivers that use only right turns.

Results from Hummer’s study can be used by transportation departments and highway design experts to determine when the diverging diamond is the best choice for a new interchange. The study identified specific collision modification factors (CMFs) that can be plugged in by planners in state and local decision-making processes.

The new freeway interchanges, which prioritize safer left turns, are also known as double crossover diamonds. Traffic that approaches a cross-freeway bridge reaches a signal-controlled gentle curve to the left that places their lanes on the “wrong” side of the bridge. Drivers turning onto the freeway ramp get a turn with no opposing traffic, while through drivers reach another modest curve to the right and a traffic light that takes them back to the “right” side of their road to continue on their way.

“It is easy to drive; most drivers never know they’re on it because we’ve got barriers and glare screens that make it easy to concentrate on the lanes. As you drive through, here’s a curve, here’s a signal, you’ve got another curve and signal on the other side of the bridge. Most drivers cruise on through,” says Hummer. “It’s not a universal solution, but it’s a good one in the correct place.”

Diverging diamonds aren’t for every freeway interchange, he notes. But where left-turning traffic is dense and interchanges installed in earlier decades need replacement or repair, the diverging diamond should be considered.

The safety evaluation of the initial diverging diamond installations was funded by the Federal Highway Administration. Another phase of the study involved six months’ worth of video monitoring at five of the test interchanges to determine how drivers responded to the new traffic pattern. During those 30 months of observation, there were very few wrong way movements or other driver errors. At several sites there were so few driver errors that the team could not construct a statistical model.

During the crash analysis at the seven test interchanges, the team examined a cumulative 28 years of crash data pre-installation and 19 years of data post-installation. As part of the project, two Wayne State civil engineering graduate students analyzed 3,000 crash reports. 

“The students did the bulk of the data collection, which was very tedious work, looking at PDF files of most of those crash reports,” Hummer says. Crash data are very messy and error-prone, and so they had to basically manually screen every one of those images.” 

The safety evaluation project began at North Carolina State University, where Hummer was teaching at the time, and involved researchers from the University of North Carolina and the University of Missouri. Hummer’s co-principal investigator on the project is Christopher M. Cunningham, program manager for the Institute for Transportation Research and Education at North Carolina State University.

The first diverging diamond intersection in the United States was built in Springfield, Missouri, in 2009. Today there are more than 50 such installations across the country, with many more planned.

Hummer is a specialist in analyzing traffic safety and efficiency innovations that include the so-called “Michigan left turn,” and superstreets. A Michigan native, he came to Wayne State University with a 20-year record of research leadership in traffic safety operations, highway design and highway safety. He has published more than 90 journal articles, participated in more than 50 funded studies, and is an internationally recognized expert in unconventional intersections and interchanges.
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Story by Tim Moran, Ph.D., part-time faculty in the Department of History and contributing writer at the College of Engineering.