Wyn Hughes, Graham Amis and Andy WalfordEnvironment and Transport Department, Cambridgeshire County Council
Cost: 29,736
Published: September 1997
This second report on 'A' class rural roads focused on the subject of traffic accidents on two lane dual carriageways in Cambridgeshire. Eight lengths of dual carriageway were selected for examination on the basis of their design standard, design speed and traffic flows. The data available for the study consisted of eight years' STATS19 report data (1988 to 1995), road environment information (inclusive of maintenance and speed data), and a driver questionnaire. Ten databases were generated around the above information, and interactions between the data were explored using statistical models.
Nearly two-thirds of accidents occurred on the links between junctions. Here, accident frequency was found to increase as major road traffic flow, the proportion of open offside aspect, the number of gaps in the central reservation, and bendiness increased. Accident frequency was also found to increase in the 5 km interval leading up to an at-grade roundabout, with stacking accidents being particularly prevalent. A decrease in accident frequency was found as the proportion of central safety barrier, nearside and offside kerbing, and offside hardstrip increased.
An examination of different junction configurations revealed that at-grade roundabouts had the highest accident rates followed by staggered T-junctions. Junctions without gaps in the central reservation had fewer accidents than those with a gap, but where a gap was present, the severity of accidents was reduced if a dedicated right turn lane was present. At grade-separated junctions, free-flow T-junctions and at T-junctions with and without gaps, accident frequency increased as the proportion of side road flow increased.
At both on and off-slips, an association was found between accident frequency and the vertical alignment of the slip road, the distance to the next junction and the verge width on the offside of the slip road. At on-slips, accident frequency reduced as the merging length increased, while at off-slips, accident frequency decreased with the presence of a hedge on the nearside of the diverging lane. However, when vulnerable road users (pedestrians and pedal cyclists) are involved in accidents at both these locations, the odds of an accident being fatal or serious is much higher than when other vehicle groups are involved.
As found in the preceding single carriageway study, the majority of drivers were experienced and familiar with the road on which the accident occurred. While few found fault with the road environment, or with their own driving, many were prepared to blame the other driver. Women and those aged over 60 had a proportionally higher involvement as the drivers of right turning vehicles in accidents, but they did not rate this task as any harder than males, or other age groups. Once again, this is a cause for concern!
From an engineering viewpoint, this study demonstrates that the more recently engineered dual carriageways have the best safety record. At junctions, the simplest configurations are also best. However, the effect of at-grade roundabouts needs to be closely monitored, and it is vital that maintenance standards are maintained. There will always be opportunities for road modification with the aim of improving safety, and excellent local returns can be shown.
Two separate studies of accidents on rural roads have now clearly shown that the drivers are the weak link in the system. Significant accident and monetary savings could be achieved through improvements in driving skills and training. It is recognised that this is a major undertaking, but the issue needs to be faced.
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