Study finds two types of MnPASS lanes equally safe
In a study evaluating the safety and mobility of Minnesota’s MnPASS lanes, researchers at the U of M’s Minnesota Traffic Observatory (MTO) found that facilities on I-35W and I-394 are performing equally well in spite of their different designs.
On I-394, MnPASS uses a restricted access design. Most of the MnPASS lane is separated from the general purpose lane by a double white line, and there are only specific points where a dashed line allows drivers to enter or exit. In the open design on I-35W, drivers have many more opportunities to enter the MnPASS lane, with a double white line preventing access only in limited locations.
The study, sponsored by the Minnesota Department of Transportation (MnDOT), was conducted in response to objections about the use of open access systems like the one on 35W. Critics say that by allowing more access points, open systems can cause more congestion and increase the risk of crashes.
“When the MnPASS lane and the general purpose lane are moving at the same speed, it’s relatively easy to change lanes,” explains John Hourdos, MTO director and the study’s lead investigator. “But when the general purpose lane is congested, jumping from 10 mph in the general purpose lane to the MnPASS lane that’s traveling at 55 mph or more can be risky—you need a huge gap. This is what open access critics don’t like.”
To evaluate the two designs, the researchers collected video data and examined lane changes in areas where drivers are allowed to access MnPASS lanes on both 394 and 35W. The team then identified and analyzed shockwaves—areas of suddenly stopping or slowing traffic—to help them measure mobility and safety.
How often shockwaves occurred was correlated to mobility, since shockwaves disrupt the good service and high speeds drivers expect in MnPASS lanes. Shockwave length was used to evaluate safety—the more vehicles that have to unexpectedly slow down or stop, the higher the likelihood of a crash.
Results indicate that shockwave activity is similar between the two systems. In fact, shockwave length was slightly longer on 394 than on 35W.
“What we found is that each system works well on its respective freeway,” Hourdos says. “On 394, the MnPASS access ‘gates’ attract more lane changes, but it works because 90 percent of the demand comes from three distinct interchanges. Open access on 35W gives better service to that freeway because demand is more spread out and interchanges are more frequent.”
“It’s valuable for us to know that both restricted and open access for MnPASS performs well ,” says Brian Kary, freeway operations engineer at MnDOT’s Regional Transportation Management Center. “MnDOT will use these results to inform the design of future MnPASS corridors.”
To help MnDOT plan for changing needs on existing MnPASS corridors, the research team also created a tool that can predict how increasing traffic levels could affect shockwave activity. For example, if an increase in traffic is expected to cause disruptions, the tool can help determine where double white lines should be installed to maintain service and performance.