Size and sign of time savings

INTRODUCTION

The conventional approach in the U.K. has been to value all travel time changes at a constant rate regardless of their size or direction. This ‘constant unit value’ approach was supported by the 1980-86 UK DoT Value of Time Study (MVA/ITS/TSU, 1987). However, there has always remained a vocal body of opinion critical of this approach (see Welch and Williams, 1997, for references and discussion). Some of the main objections have been the following:

i. small amounts of time are less useful than large amounts;

ii. small time savings (or losses) might not be noticed by travellers and any that are not noticed cannot be valued by those affected and so should not be valued by society;

iii. small time savings are said to often account for a large proportion of scheme benefits, so that small errors in measurement might mean that the scheme is really of no benefit to anyone;

iv. allowing small time savings to have ‘full’ value is said to inflate the measured total of benefits and so lead to schemes (often road schemes) being wrongly found to have sufficient net benefit to justify implementation;

v. time savings are less highly valued than are time losses, according to surveys, and so should have a lower unit value when evaluating schemes.

Both aspects relate to the possible non-constancy of the value of time for a given journey made for a given purpose (clearly, it is much less controversial, and indeed standard practice, to allow for variation by purpose and traveller type).

The practical difficulties are twofold. On the one hand, it is difficult to overcome the lay reaction that small time savings have little or no value, as well as the feeling that losses are more important than gains. On the other hand, if these points have any empirical relevance, they cause major problems for the cost-benefit calculus, as losses and gains will not cancel out, and time savings cannot be directly aggregated.

Although they do not recommend that values differentiated by size and sign should be used for appraisal, the HCG/Accent (1999) Report (AHCG) notes that [p 259]

"For any level of variation around the original journey time, gains (savings) are valued less than losses. For non-work related journeys, a time savings of five minutes has negligible value."

A recent paper by Gunn (2001) notes that corroborative results are available from a reanalysis of the 1988 Dutch value of Time study.

For reasons which will be carefully rehearsed in this paper, we do not believe that the conclusion on the differences between gains and losses is safe. This is based on an extensive re-analysis of the AHCG data. We have found it harder to reach a conclusion on the issue of small time savings, we agree with AHCG that their data undoubtedly implies a lower valuation: we have some concerns, nonetheless, as to the interpretation which should be placed on this.