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District variations in road curvature in England and Wales and their association with road traffic crashes

Robin Haynes , Andy Jones , Victoria Kennedy

Bends in roads are known to cause road traffic crashes, but do areas with many road bends have more collisions than areas with straighter roads? A geographical information system was used to generate indicators of average road curvature from a road network dataset of England and Wales at the local authority district level. The indicators were the number of bends per kilometre, the ratio of road distance to straight distance, the proportion of road lengths that were straight, the cumulative angle turned per kilometre and the mean angle of each bend. Generally the five measures were associated. Road curvature was highest on minor roads and least on major roads, and metropolitan districts had straighter road networks than non-metropolitan districts. Counts of the number of road traffic crashes resulting in fatalities, serious injuries and slight injuries in each district were obtained from Police “Stats 19” records. The association between each of the curvature measures and the number of fatal, serious and slight collisions in each district was determined by Poisson regression. Collision numbers were negatively related to road curvature after adjusting for other risk factors, so districts with straighter roads had more crashes. The cumulative angle was the curvature measure most strongly related to fatal and serious road crashes. An increase of one degree per km was associated with approximately a 0.5% reduction in crashes, enough to explain a two-fold difference in collision rates over the range of the data. Separate analysis of crashes on major roads, B roads and minor roads confirmed the conclusion. Although individual road bends may be hazardous, these results show that road curvature at the district scale is protective.

Environment & Planning A, 39, 1222-1237

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