Traffic Markings May Be Brighter on the Other Side, Study Shows

A new study from North Carolina State University shows that painted road markings, such as the lines separating traffic lanes, are significantly better at reflecting headlights in the direction that the paint was applied. This finding will help determine how states comply with new federal safety regulations and save money on painting their roadways.

The NC State researchers found that “retroreflectivity values” are higher in the direction of paint striping. “In other words,” study co-author Dr. Joe Hummer says, “the paint reflects more light if you are following the painting truck than if you are driving from the other direction.” This is a big deal, in part, because approximately 60 percent of the nation’s roads are marked with paint, rather than thermoplastics or other materials. Hummer notes that paint “is expensive – two to three thousand dollars per mile of road.” Hummer is a professor of civil engineering at NC State. His co-authors are fellow professor Dr. William Rasdorf and Ph.D. student Guanghua Zhang.

Hummer says the research could help state transportation departments better predict how long the painted markings will perform properly – and when they’ll need to be replaced. If a state can re-paint road markings every three years instead of every two, it will save a lot of money, Hummer says.

In addition, the new research could play an important role in helping state and federal transportation authorities determine how to measure compliance with upcoming standards on pavement marking brightness. The Federal Highway Administration is expected to release the standards soon, but this new research raises the possibility that a painted line in the road could pass a new brightness standard going in one direction, but fail it if tested from the other direction. This is a question that will have to be resolved in order to ensure uniform compliance with the new standards.The difference in retroreflectivity, depending on the direction one is driving, is equivalent to about a year of wear and tear. In other words, a painted line in the road will look one year newer if you are driving in the same direction that the paint was applied.

Hummer explains that this happens because glass beads are scattered onto freshly painted traffic markings in order to make them reflective. Because the painting truck is moving, those beads tend to bounce and roll before coming to rest. “The beads skid and build up paint on one side,” Hummer says. “Therefore, they are less reflective in that direction.”The research was funded through federal aid money for statewide planning and research from the North Carolina Department of Transportation. The issue is particularly important for North Carolina because over than 80 percent of marked roads in the state are marked with paint – significantly higher than the national average. The research was published in the journal Public Works Management & Policy.

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Note to editors: The study abstract follows.

“The Impact of Directionality on Paint Pavement Marking Retroreflectivity”

Authors: William J. Rasdorf, Guanghua Zhang, Joseph E. Hummer, North Carolina State University

Published: Feb. 3, 2009, online Public Works Management & Policy

Abstract: This article addresses paint centerline pavement marking retroreflectivity. The authors collect yellow centerline retroreflectivity data on two-lane highways, taking measurements in two directions. A paired t test on the data shows that there are differences and they are statistically significant. A field study is initiated to investigate the relationship between the direction the marking is painted and the retroreflectivity direction. The key result of this study is that paint centerline retroreflectivity values measured in the direction of paint striping are significantly higher than values measured in the opposite direction. Findings indicate that it is the lower retroreflectivity values of yellow centerlines (measured in the opposite direction from paint striping) that should be used to compare with the newly proposed Federal Highway Administration minimum standard because the drivers in that direction experience lower marking retroreflectivity at night. The implications for safety, for policy making, and for transportation asset management are significant.