I attended recently the UK Energy Research Centre, based at Imperial College, for the launch of a substantial literature review of what policies are effective in reducing carbon emissions from surface passenger transport.  The study is concerned with both reducing emissions from cars and reducing the use of cars.  Both approaches depend on developments in technology and on behavioural change. 

Much of the available analysis considered is concerned with vehicle regulation, vehicle and fuel taxes, mode switch and road pricing.  Relatively less attention has been given to travel planning, support for non-motorised modes and other such behavioural measures.  Moreover, significantly more of the evidence was derived from modelling and projections than from empirical assessment after implementation.  The problem is that the need is to measure the reduction in carbon emissions.  This can’t be done directly but only estimated from changes in the volume and composition of traffic, but such changes can arise from a wide variety of developments, quite apart from the particular policy or operational intervention.  So it can be hard to be sure just what is being achieved.

The report is generally thorough and well balanced, giving proper recognition to ‘rebound effects’, ie unintended consequences of interventions that result in higher carbon emissions than hoped for.  However, too little attention is given (in my view) to the implications of the constancy of average travel time (discussed in The Limits to Travel).  This implies, for instance, that policies to get people out of their cars can be expected to have constrained impact because the resulting slower travel speeds would reduce access and choice of destinations, which would be  unpopular.