Use of Pavement Markings to Reduce Excessive Traffic Speeds on Hazardous Curves

ITE JOURNAL / SEPTEMBER 1998 DESPITE PROGRESS IN REDUCing motor vehicle crash deaths and injuries in the United States, sharp horizontal curves situated on narrow twolane highways—a common roadway design feature—pose a significant safety threat. According to data published by the National Highway Traffic Safety Administration, nearly 80 percent of all fatal crashes in the United States occur on two-lane roads.1 Motor vehicle crashes on curved roadway sections occur more frequently and tend to be more severe than those on straight sections. About 40 percent of fatal roadside crashes occur on curves.2 Troxel et al. analyzed data from the Fatality Analysis Reporting System and National Automotive Sampling System and reported that more than twice as many occupants were involved in crashes per kilometer of curved road compared with straight sections.3 Crashes on curved roads resulted in nearly three times as many fatalities per kilometer as crashes on straight sections. Crash types that occur disproportionately on curves include collisions with fixed objects, head-on collisions and crashes involving rollovers.4 Viner reported that run-off-road crash risk on rural two-lane roads increases with degree of curvature.5 A study of 300 fatal roadside object crashes found a significant overrepresentation of road locations with curvatures greater than 6 degrees and negative gradients of 2 percent or steeper.6 Excessive traffic speed is a significant factor in crashes on curves and in the severity of these crashes.4 Vehicles traveling at high speeds on curved road sections not only risk departure to the outside of the curve but also frequently transgress into the opposite lane—a major cause of crashes.7 The speed at which a vehicle enters a curve is related more to the speed of the vehicle as it approaches the curve than to the sharpness of the curve.7 Therefore, efforts to reduce traffic speeds on curves should concentrate on the tangent sections preceding the points of curvature. Limited research has been published on the feasibility of installing pavement markings in advance of hazardous curves to reduce excessive traffic speeds. In two separate studies, experimental transverse pavement markings, which were intended to create an illusion of acceleration and prompt drivers to slow down, were placed across roads in advance of curves.8,9 These marking schemes were associated with significant reductions in traffic speeds. For example, Agent reported that average traffic speeds were reduced from 41.3 to 33.9 miles per hour (mph) one week after markings were installed; six months after treatment, the average speed was 34.8 mph—16 percent less than that observed during the baseline period. Rockwell et al. also reported reductions in traffic speeds, most notably high speeds, resulting from pavement markings designed to make the roadways appear narrower at the beginning of the curves.10 Significant reductions in crashes were associated with yellow bar pavement markings installed perpendicular to the direction of travel at 42 sites in advance of roundabouts and termination of high-speed roads; 52 percent fewer crashes were reported in two years following treatment, and crash benefits were sustained four years after treatm ent at seven sites for which data were available.11 This feature reports on a research experiment to examine the effectiveness of special pavement markings intended to reduce excessive traffic speeds at rural and suburban two-lane roadway locations with sharp horizontal curvatures. Use of Pavement Markings to Reduce Excessive Traffic Speeds on Hazardous Curves