HighLight Heading
June 2007
Special Research Conference Issue: Concurrent Sessions
Relocating merge area on I-94 reduces multi-car pileups
John Hourdos
Last October a double white line was painted on a stretch of I-94 in downtown Minneapolis to guide merging behavior. The new striping extends the acceleration lane of an incoming double ramp by roughly 700 feet.
Despite a constant stream of violations, said John Hourdos, director of the Minnesota Traffic Observatory (MTO), the markings have shown some success. (The MTO is part of the Intelligent Transportation Systems Institute at CTS.)
The most important effect of the markings is that crash severity has been reduced, Hourdos said. Five- and six-car crashes have been eliminated, and the number of three- and four-car crashes was greatly reduced. The right lane flows more smoothly, reducing the likelihood of multi-vehicle crashes. And congestion in the right lane shrank about 30 minutes before and after the peak period.
“In respect of crash location, we see that the system has reduced the crashes at the originally high point of Portland [Avenue] and upstream,” Hourdos said. “Unfortunately, it pushed those crashes a little bit further, closer to the new merge point.” At this time it’s unknown if this is a permanent effect or if it can be remedied with greater enforcement.
Signs warn drivers they’ll risk a $140 fine if they cross the new road markings. Yet drivers continue to do so, Hourdos said, aware that enforcement is difficult without room for a trooper to park. Mn/DOT plans to install an Interstate Grade Curb System on top of the double white lines as a possible permanent solution. These posts are designed to take hits of up to 70 miles per hour and pop back unharmed, but will dent a car as it crosses.
At certain times of the day, Hourdos said, the flow at this crash-prone area reaches 2,700 vehicles per hour, per lane, resulting in “very dense, very fast-moving traffic.” Mn/DOT turned to the MTO several years ago for an analysis and possible solutions. For the work, the MTO designed and employed observation stations on rooftops on both sides of I-94 westbound, allowing video coverage across this entire area.
After reviewing the video—which captured 95 crashes in about a year—MTO researchers determined the core problem: a shockwave from the downstream merge moves backwards, meeting oncoming traffic at speeds between 5 and 25 miles per hour. The shockwave was caused by drivers merging too soon, despite space further ahead beyond a curve.
“They do not appreciate the available space that is downstream of this curve, so they’re actually forcing themselves to the right lane, causing back-ups,” Hourdos said. As vehicles slow down to merge, they also slow traffic in the right lane. “And that is the wave that is propagating backwards approximately 1,000 feet and generates the crashes,” he said. This effect happened in both lower and higher traffic conditions.
The most affordable and easiest solution for Mn/DOT was “to change the merge location of the downstream entrance by extending road markings,” Hourdos said. “So while earlier the dotted line started approximately 500 feet before the curve, now this double white line is reaching out 200 feet after the curve.”
The MTO is continuing its study of the area.