Evaluating personal travel planning: If it is prohibitively expensive to get a robust answer then what should we do?

A recurring problem relating to the robust measurement of the impacts of personal travel planning (PTP) is identified, in terms of unrealistic expectations on the part of sponsors and, more generally, a limited understanding of the complexity of statistical inference. A number of pragmatic responses is proposed: practitioners and academics should attempt to spread a better understanding of this complexity; this "community" should also engage those who commission PTP in a debate about the need for monitoring and its purpose; the community should openly accept that PTP is not yet a mature intervention; efforts should be made to convince those who commission PTP to appreciate the value of research conducted to understand PTP's impact as opposed to simply measuring it; and the PTP community should participate more actively in the drive to improve methods used to gauge behavioural change in travel resulting from the intervention.