When comparing roads or areas that have substantial differences, looking at counts of collisions, casualties, or collision-involved drivers alone can give a misleading picture. For example, longer sections roads are more likely to have more collisions occur on them. Likewise, more heavily trafficked roads have more vehicles exposed to risk, which may lead to higher collision numbers.
Road safety analysts often use the term Collision Density to refer to the number of collisions per kilometre (or mile) of road. To make these metrics of risk more readable, Collision Densities are sometimes expressed per ten or hundred kilometres. Likewise, one can measure Casualty Densities, or Collision-involved Vehicle Densities.
The term Collision Rate is often used to refer to the number of collisions relative to traffic exposure, for example the number of collisions per kilometer travelled by vehicles along a road. As with Collision Densities, Collision Rates are sometimes expressed per million or billion vehicle kilometers travelled, to make them more readable. Likewise, one can measure Casualty Rates, or Collision-involved Vehicle Rates. It is also reasonable to adjust the traffic exposure for individual road user groups, for example looking at collision-involved pedal cyclists per million kilometers cycled, or pedestrian casualties per million kilometers walked.
Collision Densities and Collision Rates provide a more suitable basis for comparing risk as they compensate for the notable variations in the length and flow of roads or areas. Collision Density measures risk from the point of view of the road: how many collisions are expected to occur on a fixed length section of the road? Collision Rate measures risk from the point of view of the road user: what is the likelihood of being involved in a collision when driving down a fixed length section of the road? Both are useful measures of risk when deciding where to prioritise road safety interventions; having a high collision rate indicates that individual road users are more likely to be involved in collisions, but when combined with a relatively low collision density the potential impact of an intervention is limited.
Access to high quality exposure data is often a limitation in measuring Collision Rates. For most traditional data sources, such as Department for Transport Traffic Count Points, coverage is limited. Collision Density, however, can be calculated wherever road lengths can be calculated, often across the entire road network. (View Department for Transport Traffic Count Points here.)
In most cases higher traffic flows on roads increase the exposure of road users, and therefore increase the chances of a collision occurring. However, this relationship is unlikely to be linear. High traffic levels can sometimes lead to suppressed speeds which can reduce risk. Likewise, lower traffic levels on roads can give drivers the opportunity to drive faster than is safe given the conditions of the road, elevating risk. Roads with higher travel demand also often have a higher quality of engineering which reduces risk. It is therefore important that, when comparing the Collision Densities and Collision Rates of roads, these conflating factors are taken into account.
Collision Rates are often included in the Department for Transport’s annual report, Reported Road Casualties Great Britain. For individual roads, the Road Safety Foundation publish a map of Collision Rates and Collision Densities alongside their annual risk mapping report. To see the map of Collision Rates and Densities click here.