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Collective Risk Maps show the density, or total number, of crashes on a road over a given length. Crashes measured in this map are the result of interaction between all elements of the road system—road users, vehicles and roads. Collective risk is calculated by dividing the number of casualty crashes on a link by the length of the link. Collective risk maps measure risk in terms of average annual casualty crashes per kilometre.
Individual Risk Maps show casualty crash rates per vehicle kilometre travelled. This is the risk rate for individual drivers. Individual risk is calculated by dividing the frequency of crashes by the distance travelled by vehicles on each road link. Individual risk maps measure risk in terms of average annual casualty crashes per 100 million vehicle kilometres travelled.
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AusRAP has initially focused on rural road links of the AusLink National Network, defined as those outside metropolitan areas, and significant country towns. The links generally have a speed limit of more than 90km/h, though some lower-speed limit sections where these form an integral part of higher speed inter-urban routes are included. Using this definition of rural road links, we have analysed some 20,000km of road.
The highways were split into links using three criteria. First, ideally they should have a minimum 5-year total of 20 crashes resulting in death or injury over the five year period from 1999 to 2003 (for statistical reliability). Second, they should be meaningful and distinct to road users (that is, start and end at identifiable locations). Third, they should have broadly similar characteristics, such as single lane or dual carriageway, over their entire length.
Having calculated the risk of a casualty crash for each link, the links are then ranked from the lowest risk rating to the highest risk rating. The links are then allocated to one of five bands (quintiles) such that the first 20% of links are allocated to the low risk category, the next 20% to the low/medium risk category and so on up until the top 20% of links with the highest risk are allocated to the high risk category. The five risk categories are also colour coded and these colours are used in the maps to highlight the different levels of risk.
Risk Ratings
We have ranked road links according to fatal and injury crash rates (1998-02).
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Each risk map "tells a story" and provides important information about collective and individual risk for every highway. For example, some roads that carry very high traffic volumes like the Hume Highway in New South Wales tend to score poorly on the collective risk map because, in absolute terms, a large number of crashes occur on the highway. However, the same road might score well on the individual risk map—particularly on a divided section of road—after traffic volume is explicitly taken into account.
Hence, the collective and individual risk measures are most useful when used together to "tell a combined story." Roads that score poorly on both measures—those having high collective and high individual risk—should be considered as candidates for investment. The Road Protection Score and further cost-benefit analysis will assist in determining the appropriate road treatment and priority.
However, risk cannot be eliminated from roads through infrastructure improvements alone. Nor should it be. The road user must always share responsibility for a safe road system. The AusRAP risk maps help to make the connection between infrastructure and personal responsibility by highlighting sections of road where improvements are warranted, but also where road users may need to take extra care to minimise their risk until road improvements are made.
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Copyright Australian Automobile Association 2007 |
