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.
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.
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.”
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.
Read the full article in the August issue of Catalyst.