ISSUES IN EVALUATION: A JUSTIFICATION FOR AWARDING ALL TIME SAVINGS AND LOSSES, BOTH SMALL AND LARGE, EQUAL UNIT VALUE IN SCHEME EVALUATION

The conventional approach of value-of-time (VOT) studies in the UK is the 'constant unit value' approach, which values all travel time changes at a constant rate whatever their time or direction. Some major objections to this approach are that: (1) small amounts of time are less useful than large amounts; (2) travellers do not notice small time savings or losses; (3) small time savings are often claimed to explain a large proportion of scheme benefits; (4) allowing small savings to have full benefit inflates the measured total benefit; and (5) time savings are less highly valued than time losses. This paper aims to show that all these objections are plausible but flawed. The author has refined his arguments, presented here, in discussions with many individuals. The paper first presents some relevant theoretical considerations. It then gives a detailed mathematical treatment of the 'threshold argument', which appears not to be easily available in the literature and not to be obvious to everyone. Consideration is given to how large an adjustment might reasonably be made to account for the fact that, proportionally, small time savings are less precisely known. Finally, specific answers are given to the five objections discussed. For the covering abstract, see IRRD E100587.