Single-carriageway A-roads in rural areas are among the most dangerous routes on the network. Alongside journey time savings, safety improvements are therefore one of the main benefits claimed to result from quality improvements to rural single-carriageway roads. Existing appraisal methods support the engineers’ natural conviction that a better-designed road is a safer road. However, this contrasts with anecdotal evidence that as speeds and traffic flows increase, so does the likelihood of a fatal accident. This paper examines the safety issue in more detail and presents a novel analysis of accident data on the National Secondary Road network in Ireland. The Irish national secondary network consists of 2,700 km of single-carriageway A-roads, complementing the national primary routes. The context is similar to Scotland — the two countries face similar challenges of accessibility to peripheral rural areas. For this analysis, the 34 National Secondary routes have been broken up into 850 sections, each with a road quality score based on bendiness, carriageway width and gradient. Econometric analysis of accident rates and types by road quality identifies a number of interesting characteristics. As road quality improves, the overall accident rate falls. But the likelihood of a fatal accident increases, up to a certain threshold, before it begins to fall. This more accurate modelling of accident rates leads to a situation where road quality improvements can result in an increase in the economic cost of accidents, with major implications for cost-benefit analysis of rural road improvement schemes. (Author/publisher)
Samenvatting