Transport is often characterised as the fastest growing and most unmanageable source of GHG emissions, Aviation and road freight traffic pose a particular challenge for mitigation because of their rapid growth and apparent economic importance, Discussion of mitigation policy has centred on different ideas of what is ＇optimal＇, using engineering, least-cost arguments or economic efficiency arguments, These arguments can justify policies that might reduce transport energy use and GHG emissions in OECD countries by somewhere between 10% and 40% relative to business-as-usual levels in 2020, Deeper emission reductions are technically possible but would depend on changes in behaviour and, it appears, in preferences, Existing economic and engineering analysis aiming to identify or design ＇optimal＇ policies is based on unfounded assumptions about values and behaviour, Governments aiming to influence values and behaviour are more likely to succeed through an experimental approach than through trying to choose the ＇best＇ measures, The most fruitful way forward might be to design strategies that foster innovation in behaviour and institutions, in much the same way that many governments aim to foster innovation in technology.