Congestion-reduction measures on I-35W: How well do they work?

In an effort to combat congestion in our country’s urban areas, the United States Department of Transportation launched the Urban Partnership Agreement (UPA) program in 2007. The program infused nearly $900 million into transportation-related projects in four cities nationwide, including the Twin Cities metropolitan area. Minnesota’s projects—which include the installation of MnPASS dynamic toll lanes and variable message signs—focused on improving traffic flow in the I-35W corridor between Minneapolis and the city’s southern suburbs.

Buses leaving the 46th Street station in the median need to cross lanes to reach the next station located on the right side of the freeway.Buses leaving the 46th Street station in the median need to cross lanes to reach the next station located on the right side of the freeway. To understand the effectiveness of the traffic operations measures implemented under the UPA, University of Minnesota researchers examined three separate but related areas: the effects of a new variable speed limit (VSL) system, the impact of severe weather conditions on road safety, and the behavior and traffic impacts of bus rapid transit (BRT) operations. Their work was funded by the Intelligent Transportation Systems Institute, a part of CTS.

The U of M research team began by assessing the effectiveness of variable speed limit signs along the I-35W corridor designed to display advisory speed limits during periods of congestion. They found that drivers don’t typically comply with the advisory speed limit, but they do take it into consideration. “Drivers may use the advisory speed limit to gauge downstream congestion and prepare themselves for encountering upcoming shockwaves,” says Minnesota Traffic Observatory (MTO) director John Hourdos, the principal investigator. The congested conditions observed after the VSL system went into operation contained less severe shockwaves, he adds, representing a smoother and possibly safer traffic flow.

Project co-investigator Seraphin Abou investigated the impacts of inclement weather on road safety, focusing on the new priced dynamic shoulder lane (PDSL). Traditionally, shoulder lanes are used for emergency stops as well as for rainwater storage during heavy storms. The Minnesota Department of Transportation, as part of the UPA, opened the shoulder lane to traffic during specific times of the day. A portion of the road, however, lies in a low area that can flood during heavy rains. “Weather conditions have a significant impact on traffic safety, traffic demand, and traffic flow,” says Abou, an assistant professor in the Department of Mechanical and Industrial Engineering at the U of M Duluth. “The risk assessment tool we designed can be combined with intelligent transportation systems, risk communication, and operation control to predict causal mechanisms of weather-related crashes along the corridor.”

Finally, the research team examined the effect buses have on congestion along the I-35W corridor. “One of the biggest advantages of the I-35W BRT corridor is the fact that buses can use the MnPASS lane, guaranteeing uncongested traffic conditions,” Hourdos says. However, some stations are located in the median while others are located on the right side of the highway—meaning bus drivers must cross several freeway lanes to reach both stations.

By examining the buses’ movements and congestion impacts, researchers made several key findings. First, bus drivers underutilize the MnPASS lane, making lane changes as soon as possible after leaving the station located in the median. They also found that the bus lane changes do generate visible disturbances during moderate and heavy congestion; however, these disturbances do not seem to contribute to the breakdown of traffic flow. Finally, the researchers estimate that the underutilization of the MnPASS lane and the several lane changes needed to move between stations cause a combined delay of 12 to 19 minutes each day for all buses traveling between two stations. “There is currently discussion about moving the right-lane stations to the median, and we hope this research can help policymakers in their deliberations,” Hourdos says.