A growing body of literature aimed at policymakers as well as researchers criticises the quality of much transport research and advocates a methodological hierarchy with randomised controlled trials at the top as the solution. This article critically analyses the justification for such hierarchies, in the context of voluntary travel behaviour change programmes, whose effectiveness has been called into question. It proposes 5 criteria for the valid application of experimental methods, which suggest that such methods are only relevant to a limited range of transport research questions. It finds no valid justification for a methodological hierarchy and concludes that attempting to apply such a hierarchy would generate misleading advice for policy makers
