MnDOT Studies Human Factors to Establish a Wet Reflective Visibility Standard
The road planners and safety experts at the Minnesota Department of Transportation (MnDOT) know the safety benefits of highly visible pavement markings. One of the group’s primary goals is to provide an “appropriate pavement marking that meets or exceeds the standards defined in the MN MUTCD on all highways throughout the state, 365 days a year.” MnDOT traffic engineer Ethan Peterson and his colleagues also recognize the importance, and the challenge, of providing visible pavement marking guidance in the wide range of conditions drivers face on Minnesota roads.
To help them understand the visibility levels drivers need in wet, nighttime conditions and establish a wet reflective visibility standard in the state, MnDOT funded a study completed by Texas A&M Transportation Institute to observe human factors impacting pavement marking visibility. They based the study on a 60-year-old driver with 20/25 vision traveling at 60 mph at night in 1.5 in./hour rain (a heavy rain), and requiring a minimum of 1.8 seconds of preview time in order to safely maintain lane positioning. They then collected a representative participant group and had them first observe pavement markings under wet conditions in a rain tunnel, and then observe pavement markings under dry conditions.
Based on their findings, the MnDOT research team determined that pavement markings need to provide an in-service minimum continuous wet retroreflectivity level of 50 mcd/m2/lux to allow the minimum preview time at 60 mph in wet conditions. From this standard, they back-calculated the wet retroreflectivity level over four years to recommend an initial minimum continuous wet retroreflectivity level of 200 mcd/m2/lux. The MnDOT team believes that by installing pavement markings that meet these wet reflective standards, they can significantly improve road safety throughout Minnesota.
The Importance of Highly Visible Pavement Markings on County Roads
Roadway departure crashes continue to represent the largest share of serious crashes on our nation’s roads. According to traffic engineer Victor Lund, the issue is particularly pronounced on the county road system. For example, in St. Louis County, Minnesota, where Lund works, roadway departure crashes account for 54% of all serious crashes on county roads. In Minnesota, county roads are a significant contributor to the overall rates of serious crashes. In 2019, there were 364 traffic fatalities on all roads in Minnesota. Of these traffic fatalities, 47% occurred on the state trunk highway system and 39% of fatalities due to crashes occurred on the county road system.
The crash data supports the need for significant investments in traffic safety strategies on county roads. However, the county road system in Minnesota is vast and the challenge becomes deciding what to prioritize and where to invest limited budgets. In Minnesota, the fatal crash density on county roads is approximately 0.003 fatal crashes per mile per year. That means on average, for any randomly selected 1-mile county road segment, more than 300 years would need to pass before the next fatality would likely occur on that stretch of road. In other words, it is difficult to predict where the next serious crash will occur on rural county roads. For a traffic safety strategy to effectively reduce the number of serious crashes on county roads, it is best to implement that strategy proactively based upon a systemic approach.
To address this challenge, Lund and his colleagues in St. Louis County are widely deploying and maintaining consistent, visible road markings throughout their system—especially wet reflective pavement markings and 6-in.-wide edgeline markings. These pavement markings are a low-cost strategy that has been demonstrated to help drivers stay in their lanes and reduce the number of roadway departure crashes in their county. St. Louis County has realized this effort is essential in helping to drive Minnesota Towards Zero Deaths.
