Speed management on rural roads: the effect of pavement markings

Current New Zealand guidelines encourage the installation of painted centrelines and edgelines on rural roads. Though it is known that speed is directly proportional to both accident likelihood and severity, the effect of pavement markings on speed is unclear. This study looked at the effect of pavement markings (centrelines and edgelines) on speed. The study method involved speed surveys at matched pairs, to compare sites with an edgeline and centreline, to centreline only sites, and to compare a further set of centreline-only sites to sites with no markings. Sites within pairs were all on straight, flat, single-carriageway rural roads in country environments, with 100km/h posted speed limits. Pairs were matched for sealed carriageway width, with a tolerance 0.5m difference in width allowed within pairs. In addition, a before/after study compared speeds at one of the centreline-only sites before and after installation of an edgeline. Results showed that compared to the case of no markings, presence of a centreline increases the mean speed and lowers the coefficient of variation. For matched pair sites with significant results, the increase in mean speed observed within pairs comparing no centreline to centreline only was 12.1km/h, from 71.9km/h for sites with no markings, to 84.0km/h for sites with a centreline only. The increase in mean speed observed at pairs comparing centreline only to centreline plus edgeline was 11.3km/h, from 86.0km/h for sites with centreline only, to 97.3km/h for sites with a centreline and edgeline. A before/after study showed a significant increase in speed with addition of an edgeline to a centreline-only road of 7.8km/h. Overall, an increase in delineation generally leads to an increase in speed, and a lowering of the coefficient of variation across all observed speeds. As it is unlikely that drivers would admit to being consciously motivated by the presence or absence of a centreline or edgeline, it is likely that this effect is due to an unconscious process of some kind.