For options which change road traffic accident numbers and/or the severity of road traffic accident, the recommended approach to appraising the accident impacts should be followed during Part 2 Appraisal. The recommended approach to appraising the accident benefits or disbenefits in Part 2 (as discussed below) is consistent with the methodology set in the NESA manual (formerly DMRB Vol. 15).
Standard methodologies exist for calculating the projected number of accidents, the types of accidents and associated casualties in the do-minimum and do-something. The methods relate traffic on the road (measured by vehicle kilometres) to the number of accidents via the application of an accident rate. Accident rates (and casualty rates) for different road types are set out in the NESA manual and these should be used by the practitioners. The accident and casualty rates given in the NESA manual are equivalent to those used in the DfT's COBA economic appraisal program COBA11. It should be noted that accident rates and accident severity rates (casualties per accident) are predicted to change over time irrespective of whether or not a specific intervention is being considered. A full discussion of the accident rates etc. and the forecast changes over time is contained in the NESA manual (Part 6).
Standard cost values are attributed to fatal, serious and slight casualties allowing the monetisation of accidents in the before and after scenarios, and hence the calculation of the benefits or otherwise of an option. The standard costs per accident are given in the NESA manual. These include the casualty costs plus the costs per accident for insurance administration, damage to property and police costs for different types of accidents on different types of roads. Although average accident costs in Scotland are generally higher than for Great Britain as a whole, it is not possible to provide Scottish accident costs at the degree of disaggregation required for NESA. The accident costs in the NESA manual are therefore equivalent to those used in the DfT's COBA economic appraisal program COBA11.
The calculation of monetised accident costs and benefits of a road related option should follow well established methodology (as detailed in the NESA manual). For road related options it is generally considered best practice (and easiest) to carryout the accident analysis by undertaking a NESA or COBA assessment as NESA and COBA will apply all the predicted changes over time etc. automatically. (NESA or COBA ‘accident only' assessments can be undertaken where alternative traffic assignment models have been adopted).
However, in exceptional cases, it may be felt by the practitioner that the established methodology does not allow the full benefits of a unique and particular option to be identified or that it may overstate the likely scale of the benefits. For example, accident reductions due to work to address a particular black-spot may not be sufficiently captured by the NESA rate-based methodology, or the rate-based approach may not capture the benefits of improving crossing facilities near a school. In such cases, practitioners may, with extreme caution, undertake an additional quantitative or qualitative assessment. Any such departure from the established methodology should however be agreed with the relevant Overseeing Organisation. Care must always be taken to ensure that such an approach is based on sufficient historical data and should avoid falling into the trap of calculating benefits of improvements which are, in fact, simply due to the random nature of accidents. Further advice and guidance is contained in the NESA manual (Part 6).
STAG allows a qualitative assessment of accident benefits to be included. A qualitative assessment can be used to highlight matters including, but not limited to:
The rate-based methodology requires projections of vehicle-kilometres in the do-minimum (or reference case) and do-something scenarios. Such projections could be sourced from transport models, or may be derived from other data as appropriate for the scale and type of intervention being considered (e.g. traffic counts combined with growth rates).
For public transport options, changes in accidents involving traffic due to a transfer of trips away from cars to public transport are captured by the NESA rate-based approach reviewed above. To use the NESA approach it is necessary to develop a projection of the reduction in car vehicle-kilometre due to the option. These could be sourced from a transport model, or could be derived from other available data combined with reasonable assumptions.
