Setting retroreflectivity requirements based on the relationship between pavement-marking brightness and safety would be the ideal, but the task is monumental.
The Texas A&M Transportation Institute (TTI) says it has identified a technique for developing safety-based retroreflectivity criteria for pavement markings.
The technique quantifies the link between pavement-marking retroreflectivity and nighttime safety and is described in a Transportation Research Board paper submitted by TTI Senior Research Engineer Paul Carlson and TTI Associate Research Scientist Raul Avelar.
According to Carlson, the relationship between safety and pavement-marking retroreflectivity has been studied with mixed results. “The FHWA [Federal Highway Administration] is developing minimum pavement-marking retroreflectivity levels for the MUTCD, which are based on the visibility needs of the driver—can you see the pavement marking far enough to react in time? That’s different from what we’re doing here. We are trying to develop safety-derived retroreflectivity criteria. The visibility-derived minimums are thought to be related to safety but our research is based on nighttime crashes and measured retroreflectivity levels.”
For this study, TTI probably had the richest combination of retroreflectivity data and crashes available for research. Avelar used advanced mathematical analyses to mine and study the data, which were made available from the Michigan Department of Transportation.
“We accounted for other important factors explicitly, such as traffic volume and roadway length. We then calibrated a function describing how retroreflectivity contributes to explaining the number of crashes,” said Avelar. “We expressed it in a way that makes mathematical sense. We could then quantify the expected significance of that effect in statistical terms.”
The result was the creation of a crash modification function for pavement-marking retroreflectivity. With that, DOTs may define a point at which retroreflectivity of the centerline is most effective, from the safety standpoint. Carlson’s research team is currently in the process of acquiring additional data sets from the North Carolina DOT for further testing and validation.
“Our results indicate that retroreflectivity of pavement markings does relate to safety,” said Carlson. “It’s been an elusive relationship. We’re not saying that we’re there, but we have a very promising technique that raises the bar.”