8.2.3 Assessment of Walking and Cycling Accidents

In common with the assessment of accidents for road vehicle schemes, it is expected that the primary starting point for the assessment of accidents for walking and cycling schemes will be identifying the change in the number of users and the route type; however, Transport Scotland recognize that this is an area which is not well researched and provides significant challenges, given the variety of different facilities which could form part of a scheme and the amount of time needed to establish changes in the number of accidents which result from a scheme. Guidance in this area is the same as that provided by the Department for Transport.

Possible methods for estimating accident rates could include comparative studies of the performance of existing similar schemes combined with expert judgement. In all cases, the detail of the design might be crucial, as there are clear differences in scale and sensitivity compared to schemes for motorised users. Clearly, the monitoring and evaluation of cycling and walking schemes is important in order to provide more robust input to the analysis of further schemes.

The introduction of an intervention may also demonstrate a large enough mode shift in the modelling to produce significant reductions in accidents associated with other modes. This will have the effect of increasing the value of a scheme where fewer traffic accidents occur. Where this is the case, such considerations must be included in the appraisal. Monitoring techniques such as stated preference surveys are useful in determining potential mode shift where walk and cycle users state that an alternative mode of transport was available to them.

There is good evidence to suggest that increasing levels of cycling does not result in an equivalent increase in the numbers of accidents involving cyclists (all other things being equal). Jacobsen (2003) used American and European data to create a power function model of the type:

I = aE^b

Where:

I = injury measure
E = measure of walking and cycling
a = a constant
b = a constant and was found to be approximately 0.4

This implies that a doubling of cycling would only lead to a 32% increase in the number of cycling accidents (2^0.4 = 1.32) and that therefore the cyclist accident rate decreases. It seems intuitive that this model is applicable for cases above a certain critical mass of walkers and cyclists. For very small values, one should be careful in the application of this model as a close-to-linear increase in accidents per additional unit may well be more appropriate. The evidence base for this requires expanding through further research and monitoring.

Ideally one should incorporate the background changes to walking and cycling accident rates, which may indeed be decreasing over time. This may be due to increased bicycle safety, awareness and public information campaigns.

Where facilities are being introduced which are expected to have a significant impact on the accident rate for cyclists and pedestrians, such mitigation is likely to have a more significant local impact than any increase in these modes.

Once the accident forecasts have been completed, one can then assign economic values to those accidents in order to derive the benefits or costs brought about by the intervention. The NESA Manual contains monetary values for accidents of different severity: fatal, serious and slight (DfT, 2004d). These accident costs should be uplifted over time in line with increases in real GDP per capita, somewhat offsetting the discounting process.