Dear UTSG listmember,
The recent contributions to this list by Michael Nandris are helpful.
Measurements of (subjective)VTTS from SP studies routinely find lower
per unit valuations for small time savings than large savings, causing
some people to argue (mistakenly) that in appraisal small time savings
should be downweighted to some extent.
If it is true that "short intervals of time tend to be overestimated", then
someone receiving a 4 minute time saving might report it as 5 minutes.
Consequently, if that person were presented with a 5 minute time
saving in a SP experiment, they might imagine it as 4 actual minutes
and value it accordingly when considering if it was worth paying x
pence for. This would give a downwardly biased SP estimate for time
savings of that size.
However, if the scheme went ahead and there was a 4 minute time
saving, those users would think it was 5 minutes, and so no harm would
have been done by ignoring the SP evidence from experiments
involving small time savings and using a constant unit value of time
instead.
If you have been, thankyou for listening.
Tony Fowkes
Senior Lecturer in Transport Economics
Institute for Transport Studies
University of Leeds
Leeds LS2 9JT
0113 3435340
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