Pavement surface markings are an important safety feature on Minnesota’s roads and trails. But the characteristics that make these markings visible in daylight and reflective at night can also make them slippery in rainy or freezing weather, reversing their safety benefits.
“The frictional characteristics of a pavement’s surface are a major component of safety. Good skid resistance allows users to move and stop safely under various weather conditions,” says Mihai Marasteanu, a professor with the University of Minnesota’s Department of Civil, Environmental, and Geo- Engineering and a CTS scholar. “The sudden change in frictional characteristics when transitioning from normal pavement surfaces to pavement markings can create a safety hazard, especially for pedestrians, motorcyclists, and bicyclists.” To explore this issue, Marasteanu led a study examining the friction differentials between bare pavement and colored pavement and pavement markings.
Different marking materials are used across the state. These include paints, preformed tapes, heat-treated epoxies, methyl methacrylate, and thermoplastics that encapsulate beads, broken glass, or other filler materials—including a regional option, crushed taconite. In the study, sponsored by the Minnesota Department of Transportation (MnDOT) and Minnesota Local Road Research Board, Marasteanu and his co-investigators evaluated a range of marking products to determine their friction properties and durability.
“We didn’t want to repeat work done elsewhere, so we began with a review of existing national and international research,” Marasteanu says. The NordicCert certification process used in Scandinavian countries for selecting pavement marking products was highly relevant, considering Minnesota’s similar climate. The investigators also evaluated historical motorcycle crash data for pavement-related insights and reviewed Federal Highway Administration recommendations on pavement design.
Marasteanu’s team also surveyed pedestrians, bicyclists, and motorcyclists about their experiences with road markings. All three vulnerable road user groups surveyed reported that pavement markings were slick when wet and recommended marking material improvements, such as adding texture or roughness for better safety.
Next, the research team performed experimental friction testing on ten marking products. Using three different friction-testing devices, the researchers determined the friction differential between the pavement and the markings at MnDOT’s MnROAD outdoor pavement research facility.
Testing found that marking types with the lowest coefficients of friction (and therefore, the greatest friction differentials) are latex with embedded beads, epoxy with beads, and Preform Thermo (a type of thermoplastic marking material that comes in pre-cut shapes). Additives such as corundum, crushed glass, and taconite further enhanced the friction characteristics of markings when incorporated within the marking material.
“We were uncertain of the quantitative friction values we should strive for and how to compare skid resistance of different pavement marking products,” says Ethan Peterson, pavement marking and crashworthy engineer with MnDOT’s Office of Traffic Engineering. “This work provides foundational insight into how to compare materials going forward to enhance the safety of vulnerable road users.”
The study’s findings include user insights and friction test results as well as observations about travel speeds and wear level on markings. However, the study was unable to compare materials over long periods of time, in all weather conditions, and on every type of road geometry. (Materials used on highways, pedestrian areas, and roundabouts, for example, see different use patterns at different speeds.) Marasteanu recommends future testing of materials with finer gradations of additives and evaluating epoxies during freezing conditions.
Pavement marking materials continue to evolve and new solutions are emerging. For example, strategic marking placements at crosswalks could allow motorcycles to pass between markings and avoid encountering the material altogether.
“As we see more e-bikes and scooters, with their faster acceleration and higher speeds, become more prevalent in urban environments, designing safer roadways and pedestrian areas becomes even more critical,” he says. “We hope our work provides meaningful information on marking materials that will help engineers make the best decisions.”
—Amy Goetzman, contributing writer
