Background: numbers and risk

Casualty numbers and risk

Risk

Casualty numbers and casualty rates

An area of casualty analysis is concerned with risk. Numbers are often ’normalised’ into rates, such as ’the number of casualties per head of population’. This then allows comparisons to be made, eg, cycling compared to driving, or one geographic area compared to another. Rates can be an estimate of the possibility of injury based the amount of exposure, ie, the time or distance a person spends in an environment where they may be injured. The Department for Transport, in their annual casualty report, provide an estimate of casualties per miles travelled.

Perceived risk

Casualty rates and similar measures of risk are usually based on collision records. “Perceived risk” is often used to describe an individual’s sense of the probability of being injured on a journey they are considering making.

Numbers vs rates

The ultimate interest in casualties should be their absolute number - the closer to zero the better, regardless of the number or length of trips. The figures on this site are concerned with casualty numbers. There is, anyway, no accurate local information that would allow us to come to an accurate measurement of risk for our area. For example, we would need additional information on:

1. the number of cycle journeys;
2. the length of cycle journeys;
3. time spent cycling;
4. whether any changes in the level of cycling represent a change in the number of people cycling or a change in the habits of the existing cycling population;
5. a profile of the cycling population (and other road users), eg, whether the proportion of experienced to novice cyclists is changing;
6. the type of environment where people were cycling, eg urban-rural, road class.

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