Quantification of safety benefits resulting from road smoothing
Relationships between road safety and road roughness parameters routinely used for road asset management purposes were examined to allow identification of appropriate road maintenance interventions and their benefits to be quantified. Statistical modelling was employed to identify significant relationships between crash risk, road geometry and routinely measured roughness parameters, such as the International Roughness Index (IRI) and road profile variance. Test track and on-road measurement programmes utilising instrumented vehicles were used to establish whether isolated roughness elements were more or less safe than a uniformly rough road for cornering manoeuvres and to quantify the effects of roughness on the braking distances of passenger cars and trucks. The key findings were that (1) road roughness had an increasing detrimental impact on crash risk as horizontal curvature increased; (2) the roughness parameter shown to be the best predictor of crash risk was 10m wavelength profile variance, although it was only marginally better than IRI; (3) at speeds greater than 50 km/h, braking distances increased with increasing roughness for both passenger cars and light trucks; (4) during cornering, roughness effects were greatest when encountered at the apex of the curve and on all four wheels; (5) a significant reduction in tyre contact occurs at a critical speed that depends on the elevation profile of the road surface; and (6) smoothing of low volume rural roads was shown to be a cost effective safety intervention whenever the existing reported injury crash density exceeds 0.5 reported injury crashes per year per kilometre for straight road sections and 1.8 reported injury crashes per year per kilometre for moderate curves.