Software tool helps determine safe access points for high-occupancy toll lanes

toll road
Photo: MnDOT

High-occupancy toll (HOT) lanes are one of the many tools available to help manage traffic demand on congested urban freeways. When managing HOT lanes, roadway engineers must decide whether to allow open HOT-lane access at most points or to close access and only permit drivers to enter or exit these lanes from select ramps and access points. 

A new software tool developed by researchers at the Minnesota Traffic Observatory (MTO) allows MnDOT’s Regional Transportation Management Center (RTMC) to better design HOT-lane access for a good balance between service to the driving public and safety of the facility. The tool assumes an open design and produces safety and mobility performance measures along the HOT-lane facility. Given the desired balance, the tool also helps engineers design the optimal location of restricted access sections.

“While open access can greatly increase mobility by permitting vehicles to access the HOT lane as quickly as they are able after entering the freeway, there may be some safety concerns with allowing lane changes at any point,” says John Hourdos, MTO director. “HOT lanes tend to have lower densities and higher speeds, so vehicles crossing the lane boundary must negotiate a speed change, which can cause the vehicles approaching from behind to form a shockwave that can lead to a crash.” 

In the Minneapolis–Saint Paul metropolitan area, two major freeways currently include designated HOT lanes, and several more are planned or under construction. Until recently, drivers could only enter or exit HOT lanes from select ramps and access points. MnDOT now prefers open-access HOT lanes for the increased options they offer road users but needed a way to monitor and predict the incidence of shockwaves and the safety and mobility problems they create.

The researchers leveraged and integrated findings from three previous studies to create the new software system. It draws on historical data to identify patterns of traffic demand over time and generate predictions of points in the HOT lanes at which shifts from open to closed access will offer the most benefit. The program is designed to integrate smoothly with the RTMC’s current software and data capabilities and includes a module for access design, a module for generating data, and a web application.

“This tool delivered to MnDOT is calibrated for the Twin Cities,” Hourdos says. “It takes real-time data and diagrams each location separately for lane changes and reaction time. It took theoretical ideas and made them usable.”

The new tool has been embraced quickly by engineers at the RTMC, who plan to use the software to report HOT-lane performance and to create analyses and recommendations for changing specific locations from open access to closed access.

“Prior to this project we didn’t have a tool to assist us in designing HOT-lane access,” says Brian Kary, director of traffic operations at the RTMC. “There is a lot of debate around the country surrounding high-occupancy designs. This tool helps us develop designs and monitor existing corridors.”

The MnDOT-sponsored project also offers benefits for the broader transportation community, in which experts debate the relative merits of open- and closed-access HOT-lane designs. “With MnPASS, MnDOT has adopted the approach of an open design for HOT-lane access,” Kary says. “This software allows us to evaluate the performance of the MnPASS open design, which not only benefits MnDOT but helps to inform the national discussion on HOT-lane design.”

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