Computing what the public wants: some issues in road safety cost-benefit analysis.

In road safety, as in other fields, cost-benefit analysis (CBA) is used to justify the investment of public money and to establish priority between projects. It amounts to a computation by which 'few' - the CB analysts - aim to determine what the 'many' - those on behalf of which the choice is to be made - would choose. The question is whether there are grounds to believe that the tool fits the aim. I argue that the CBA tool is deficient. First, because estimates of the value of statistical life and injury on which the CBA computation rests are all over the place, inconsistent with the value of time estimates, and government guidance on the matter appears to be arbitrary. Second, because the premises of New Welfare Economics on which the CBA is founded apply only in circumstances which, in road safety, are rare. Third, because the CBA requires the computation of present values which must be questioned when the discounting is of future lives and of time. Because time savings are valued too highly when compared to life and because discounting tends to unjustifiably diminish the value of lives saved in the future, the CBA tends to bias decisions against investment in road safety.