A study by Texas A&M University's Texas Transportation Institute (TTI) concludes that six-inch, as opposed to four-inch, edge lines on roadway shoulders may reduce crashes and fatalities on rural two-lane highways.

An Evaluation of the Effectiveness of Wider Edge Line Pavement Markings analyzes FHWA data from three states: Kansas, Michigan, and Illinois. It shows wider edge lines reduce total crashes 15% to 30%, and fatal plus injury crashes 15% to 38%. The benefit-cost ratio for wider edge lines is $33 to $55 for each dollar spent — similar to shoulder rumble strips. Wider edge lines can offer similar safety benefits as rumble strips, but without the noise and potential pushback from the bicycling community.

The study suggests amending the Manual on Uniform Traffic Control Devices (MUTCD) to set the minimum edge line width on rural two-lane highways to six inches, so public agencies can implement the policy in a uniform and consistent manner.

The study was sponsored by the American Glass Bead Manufacturers' Association. Glass beads are commonly used in light-reflective highway safety markings.