Drivers and businesses benefit from a freeway network that is predictable and can adapt to and recover quickly from traffic disturbances such as construction, traffic incidents, and poor weather. Developing and maintaining a reliable and resilient freeway network, however, is a major challenge for transportation engineers. Doing so requires identifying the corridors vulnerable to congestion—and as a result, unreliable travel times—and quantifying a given corridor’s ability to cope.
In a project led by UMD civil engineering professor Eil Kwon, U of M researchers analyzed and identified the most vulnerable portions of the Twin Cities freeway network and enhanced an analysis tool to better estimate travel-time reliability and operational resilience. This information is critical for prioritizing improvements and developing effective short- and long-term strategies for mitigating congestion.
In the first part of this research, the team assessed the level of congestion and its sources on Twin Cities’ freeway corridors from January 2018 until December 2023 using the Travel-Time Reliability Estimation System (TeTRES) developed in previous related research. “TeTRES integrates all kinds of data—traffic, weather, crashes, work zone, special events, and road surface condition—and allows users to estimate, for any route in the corridor and for any time period, travel-time reliability and traffic-flow measures, such as vehicle miles traveled (VMT), under any kind of operating condition,” Kwon says.
In the previous phase of the research, the team used TeTRES to estimate travel-time reliability and traffic flow for 48 direction-specific corridors (e.g., one side of a freeway or a particular travel direction) in the metro freeway network from January 2016 to September 2020.
Building on that work, this project reconfigured the metro freeway network into a total of 74 directional corridors, reflecting the specific needs of MnDOT. The researchers then estimated travel-time reliability and traffic flow on each corridor from January 2018 to December 2023 for both morning and afternoon peak travel periods. Further, the researchers quantified the corridor-wide vulnerability of routes in terms of travel-time variability and level of congestion. The resulting vulnerability index of each corridor was then used to rank the reliability of the metro corridors.
“One of the main reasons this research is important to MnDOT is that it allows us to evaluate the impacts of various types of shocks to the highway network,” says Michael Iacono, performance and investment data analyst with MnDOT’s Office of Transportation System Management and technical liaison for the MnDOT-sponsored project.
The results from this phase have also been applied to evaluate the impacts of several operational changes in the metro freeway system. Examples include MnDOT’s installation of I-35W high-occupancy toll lanes between Roseville and Blaine in 2021 and expansion of ramp metering on Trunk Highway 610 in 2023.
“This research also provides us with a means to evaluate any proposed countermeasure—like ramp metering, variable message signs, or E-ZPass lanes—that is targeted to these poorer-performing corridors to ensure that they’re effective,” Iacono says.
The second part of this study developed a corridor-wide operational resilience index (CORI) to measure the ability of a given freeway segment to cope with the delay caused by various types of traffic disturbances (weather, incidents, events) . CORI values of 74 directional corridors were estimated using data from 2018, 2019, and 2023. Further, the effects of the geometric structure of the freeway on traffic flow were also measured.
Results showed a strong correlation between corridor-wide operational resilience and geometric structure: the routes with low levels of geometric friction (caused by physical design features such as curves, lane width, or ramps) showed more resilience than those with high-friction levels. In addition, routes with strong resilience were more productive and reliable—such as having higher VMT with less variable travel time—than routes with low resilience. Finally, operational resilience during dry days was consistently higher than during rainy days for the same corridors.
The research findings could help MnDOT develop effective strategies to improve the geometric and operational environments of metro freeway corridors, design new facilities, and reconstruct and redesign existing ones, Iacono says.
—Peter Raeker, contributing writer
