Three papers outline new ways to look at interchange safety
The state of Missouri was a pioneer in adopting Diverging Diamond Interchanges, and its flagship university’s civil engineers are paving the way in studying their safety.
Faculty members and students from the University of Missouri Civil and Environmental Engineering Department recently published three papers in the Transportation Research Record, the publication of the Transportation Research Board of the National Academies, on the safety of innovative roadway interchange designs, particularly Diverging Diamond Interchanges (DDIs).
DDIs divert traffic exiting major thoroughfares by requiring traffic on an overpass or underpass to drive on the left side of the trafficway, improving safety by eliminating left turning conflicts common in regular diamond interchanges, and the traffic flow is optimized through use of traffic signals.
Professor Carlos Sun, Associate Professor Praveen Edara and graduate student Boris Claros partnered with researchers from Wayne State University in Michigan, the University of North Carolina and North Carolina State University on “Safety Evaluation of Seven of the Earliest Diverging Diamond Interchanges Installed in the U.S.” Claros, Edara and Sun also authored “Site-specific Safety Analysis of Diverging Diamond Interchange Ramp Terminals.”
Both papers analyzed several years of crash data and controlled for several variables (e.g. traffic amounts) to both account for safety of the interchanges before and after becoming DDIs and also safety based on DDI ramp lengths. For this work and several other roadway safety studies, MU civil engineering researchers studied more than 10,000 crash reports in two years.
“We actually had to calibrate to local conditions,” Sun said of the variables the team takes into account in its analyses.
“Now we can perform a safety analysis of any type of intersection in our state. We have to calibrate for traditional diamond interchanges and then go ahead and do crash reviews for all the DDIs that were in place.”
In the ramp terminal project, the team found that fatal and injury crashes decreased by up to 73.3 percent after changing to a DDI, and overall crash frequency decreased by as much as 54 percent. The evaluation of the seven earliest DDI interchanges illustrated that a DDI should reduce crashes by 33 percent on average.
Sun said the team used elements outlined in the Highway Safety Manual and previous models for the safety performance of various non-DDI interchanges and created a model for DDIs, which up until now had seen little concrete safety data produced because of their relative novelty. The first such interchange in the U.S. opened in Springfield, Mo. in 2009.
“Before our studies were released, there really was little information about the safety effects of diverging diamonds. This was leading safety research in terms of diverging diamond interchanges,” Sun said.
Henry Brown, a civil research engineer at MU, was lead author on a third paper, “Maintenance of Traffic for Innovative Geometric Designs Work Zones.” Alongside Sun, Edara and students Tim Kope and Amir Khezerzadeh, Brown investigated common practices for diverting traffic while constructing and later maintaining innovative geometric interchanges, which include DDIs, among others. Brown and the team surveyed project managers on construction and maintenance projects for a variety of interchanges to see which practices tended to work the best and poured over construction plans to establish a set of best practices for the various interchanges.
“DDIs, roundabouts and a few other types — a lot of them, you’re retrofitting to make this new facility and changing traffic patterns,” Brown said. “There’s not a whole lot of guidance out there.”
What the team discovered is that, because closing the entire road for an extended period usually isn’t practical, crews build the interchanges in phases, then close briefly — say, for a weekend — to put in signage to alert drivers to the change in traffic flow.
“If you’re retrofitting a diamond interchange, you’re actually completely changing the traffic pattern when converting it to a DDI,” Brown said. “You can’t just in five minutes switch people to driving on the other side of the road. You close for the weekend and put in the signage.”