Lines, signs and symbols painted on the pavement play a major role in providing drivers with needed information about how to navigate the roadway safely and legally. In order to ensure that drivers can see the markings at night, the paint is mixed with micro-sized glass spheres, making it retroreflect the light from vehicle headlamps to drivers’ eyes. But as this paint–glass bead mixture is applied to the road, degrades over time, and is reapplied, what effect does it have on the people handling it and on our environment? Researchers with the Texas Transportation Institute (TTI) and the Texas A&M University Zachry Department of Civil Engineering (CE) recently began an effort to find an answer to this question.

Mixed with paint, microscopic glass beads like those seen here help enhance the retroreflective property of pavement markings.

The microscopic glass beads added to pavement paint most often start out as recycled glass feedstock, which can have high levels of arsenic and other heavy metals.

“In the past, arsenic had been used to purify glass. While we no longer purify glass this way, arsenic is still present in recycled glass that becomes the beads,” says Bryan Boulanger, assistant professor in CE. “Volume-wise, a lot of glass beads go down on the roads, and they are constantly being replaced.”

“I estimate that there are about 80 million pounds of glass beads used each year on U.S. highways,” says Paul Carlson, head of TTI’s Operations and Design Division. With such a large quantity in use, private producers and public officials began to wonder if the beads could leach heavy metals into the ground or affect human health.

So the Federal Highway Administration (FHWA) tasked Boulanger and Carlson to find out the concentrations of heavy metals in the beads. After collecting samples from around the country and participating vendors, the beads were ground down to measure the metal contents and determine what chemical forms could leach out. Researchers also observed how the glass beads are handled in the workplace to see what risks there might be to the workers. Since the glass beads are approximately the size of small ball bearings, workers could inadvertently consume them through unwashed hands.

The statistics gathered were incorporated into a risk assessment model that will be used by decision makers at all levels of transportation. The model is currently being reviewed for impartiality and refined for accuracy. An analysis of small samples of glass beads shows only a weak relationship between the metal contents and the retroreflectivity level.

“Glass beads are a very integral part of highway safety. So when considering the risk associated with heavy-metal contents in the beads, decision makers have to balance that with the risk of not having the beads in the paint,” says Boulanger. More research is needed to determine the full impact on pavement marking retroreflectivity, if any, as well as to assess how removing metals from the glass beads will affect their efficacy.

This Issue

Making the Grade: Tomorrow’s Transportation System

Volume 48, Number 1
March 2012
Issue Overview