|The number of fatal and serious road accidents as a proportion of all road accidents involving injury is approximately 40% higher in Scotland than in England and Wales. The Scottish Office commissioned The MVA Consultancy to explore the reasons behind this higher severity rate and to investigate whether the difference was caused by variations in the type of road network in both countries. As the severity variance had previously been identified as mainly a rural problem, the study was restricted to an analysis of accidents on trunk and principal roads in non-built up areas.|
- The study was able to account for 60% of the severity difference between Scotland and England/Wales on the non-built up trunk and principal road network in terms of different distributions on the network of accident types.
- Accidents on low flow roads are on average more severe than those on high flow roads, and estimated traffic flow at time of accident was found to account for 23% of the severity difference.
- Similarly, accidents on links between junctions are more severe; Scotland had proportionately more accidents occurring on road links and this accounted for a further 20% of the severity difference.
- Road type accounted for 10% of the severity difference, with Scotland having proportionally more single carriageway roads and more fatal and severe accidents occurring on them.
- Other network variables tested, including bendiness, number of intersections and type of junction, did not contribute significantly to the differences in severity distribution between Scotland and England/Wales.
- Non-network factors such as differences in weather and light conditions, response time of emergency vehicles and size and/or age of cars were not tested but may in part explain part of the residual 40% difference in severity rate.
|The number of fatal and serious road accidents as a proportion of all road accidents involving injury - 'the severity rate' - is approximately 40% higher in Scotland than in England/Wales.|
|Although this has been the case for many years, the reasons have not been fully understood. The Scottish Office commissioned an earlier study, undertaken by the Institute of Transport Studies (ITS) at The University of Leeds, to review the literature on the subject of accident causation and severity, to conduct preliminary analysis of such summarised accident data as were available and to examine the feasibility of investigating the issue further 1. One of the conclusions of this study was that, once pedestrian accidents were removed, the higher severity rate problem in Scotland was largely a rural one.|
|From their literature review, the ITS report listed a number of variables which have been identified by previous research as affecting accident severity. These were: |
- road type (motorway/dual carriageway/single carriageway);
- level of traffic flow;
- frequency of intersections;
- average vehicle speed and speed variance;
- horizontal curvature;
- carriageway width;
- level and quality of lighting;
- driver visibility/sight distance; and
- median carriageway width.
|It was hypothesised that differences in the distribution of these network variables between Scotland and England/Wales could explain the overall severity difference between the countries. For example, it was known that accidents on single carriageways were, on average, likely to be more severe than those on motorways or dual carriageways and Scotland had proportionally more single carriageway roads than England/Wales.|
|The ITS review also identified a number of non-network variables which might explain some of the severity difference. These included: |
- differing weather conditions;
- differing light conditions;
- differences in response time of emergency services;
- differences in the level of drink-driving; and
- differences in the size and/or age of cars.
|It was recognised that further investigation of the severity difference was required and the MVA Consultancy was commissioned to carry out the research.|
|The MVA Study|
|A specific aim of this new work was to investigate the hypothesis that the severity rate difference was caused by variations in the distribution of network characteristics between the two countries. The study remit did not include detailed investigation of other factors that affected accident or severity rates.|
|When the ITS study assessed the availability of network and traffic data relating to different road types, it found that the most comprehensive and reliable information was maintained for trunk and principal roads. For this reason, and because the severity variance between Scotland and England/Wales had been identified as largely a rural one, the scope of this study was limited to the investigation of accident severity on trunk and principal roads where the speed limit was 50mph or over.|
|The theoretical basis for the study method was the assumption that accident severity will be similar for any particular subset of accidents for both Scotland and England/Wales. For example, single vehicle accidents occurring on low flow dual carriageways in Scotland would, on average, have similar accident severity rates as single vehicle accidents occurring on low flow dual carriageways in England/Wales. Any network related difference in the severity rate for all accidents must therefore be largely due to differences in the distribution of subsets of accident. In other words, Scotland must have proportionally more of those types of accident which are most likely to involve serious injury or death. The research approach, therefore, was founded on identifying different distributions of accident types between the countries, not a detailed investigation of factors that affected accident severity.|
|The study used information held on nearly 115,000 Accident Attendant Circumstances (STATS 19) forms for accidents in non-built up areas on trunk and principal roads between 1992 and 1994 inclusive. Accidents involving injuries to pedestrians only were excluded. Table 1 below shows the severity rates for the study dataset.|
|Table 1 - Severity Rates for Study Dataset|