The safety benefits of roundabouts over signalized intersections have been well-established for vehicular traffic, with previous studies showing decreases in fatal and serious injury crashes. But many stakeholders continue to perceive that heavy commercial vehicles are at risk and may overturn when navigating a roundabout. A MnDOT investigation comparing commercial vehicle crash data between roundabouts and intersections with traffic signals found fewer total crashes and serious crashes in roundabouts compared to other intersections, particularly after MnDOT modified roundabout design guidance to address commercial vehicle concerns.
Nearly 500 roundabouts have been constructed in Minnesota on local, county and state roads. The geometric designs of roundabouts reduce driving speeds, and splitter islands separate traffic that is entering the roundabout from traffic that is exiting, which decreases the number of potential vehicle conflict points. A safety evaluation performed by MnDOT in 2017 showed significant decreases in fatal (86% decrease) and serious injury (83% decrease) crashes after the installation of a roundabout.
However, the perception among stakeholders is that larger commercial vehicles have more rollover crashes in roundabouts. Minnesota’s 2020-2024 Strategic Highway Safety Plan identified commercial vehicles as an emerging priority in the state’s safety focus. Although roundabout safety was not identified as a specific strategy in the plan, MnDOT wanted to evaluate whether heavy commercial vehicles are more likely to have rollover crashes at roundabouts than at signalized intersections, and if there are common factors attributing to these crashes. Additionally, the agency wanted to assess the impacts of 2018 updates to roundabout design guidance to address heavy vehicle rollover concerns.
What Did We Do?
MnDOT investigators reviewed studies on truck and heavy commercial vehicle safety at roundabouts, evaluating how various factors relating to intersection design, vehicle configuration and driver behavior affect the rollover risk of commercial trucks navigating roundabouts.
“These results illustrate the significant safety benefits of roundabouts over signalized intersections for heavy commercial vehicles, including a distinct decrease in severe crashes, particularly since roundabout design guidance changes in 2018,” said Mark Wagner, assistant state traffic safety engineer, MnDOT Office of Traffic Engineering.
Over 100 roundabouts were chosen for the evaluation, all built by 2017. The roundabouts were located outside of residential areas on certain road types, such as state highways, where average annual daily traffic data was available for heavy commercial vehicles. For comparison, MnDOT identified 95 signalized intersections based on the same criteria with similar traffic volumes.
An evaluation of crashes within 500 feet of the roundabouts and signalized intersections focused on crash rates, crashes per million entering vehicles and crashes per million entering heavy commercial vehicles from 2017 through 2022. Also, an analysis of 181 additional roundabouts constructed from 2018 to 2023 illustrated the effects of changes in MnDOT’s design guidance for roundabouts.
What Did We Learn?
The review of research on truck and heavy commercial vehicle safety at roundabouts illustrated roundabout geometric factors that can impact rollover risk. Crowned travel lanes rather than cross-sloped lanes, where one side of the lane is slightly higher than the other, may improve stability for vehicles moving through or turning left but slightly reduce stability for vehicles turning right. Higher-curbed truck aprons can increase rollover risk as trucks mount and dismount when entering or exiting the circulatory lane. Finally, wider circulatory lanes allow more maneuvering room and improve stability.
Longer trucks with double trailers show higher rollover risk as do fully loaded trucks with a higher center of gravity. Speeding through a roundabout significantly increases rollover risk. Educating truck drivers on roundabout navigation techniques, vehicle loading strategies, and optimal speeds could decrease rollover risk.
The comparison of crashes at roundabouts built before 2018 and signalized intersections showed roundabouts had fewer total crashes and fewer fatal and serious injury crashes than intersections with traffic lights. Fifteen of 157 crashes (roughly 10%) at roundabouts were rollovers: one fatal, three suspected minor injury and 11 property damage crashes. For the roundabouts built after 2018, only 3% were rollovers, suggesting that the updated design guidance increased safety for heavy commercial vehicles.
The most frequent type of commercial vehicle crashes at roundabouts involved other vehicles sideswiping the trucks when passing, which suggests that single-lane roundabouts could lower crashes, although their narrowing circulating roadway may increase the risk of rollovers. Overall, the analysis indicates that despite an increased rollover risk, roundabouts are safer than signalized intersections for heavy commercial vehicles.
What’s Next?
This study offered valuable insight for policymakers, stakeholders and the public, demonstrating the safety benefits of roundabouts for large commercial vehicles. MnDOT will continue to use roundabouts as alternatives to signalized intersections where appropriate and ensure roundabout designs reflect the most current safety standards.
Crossroads 