Forecasting road traffic and its significance for transport policy

Traffic forecasting developed initially to decide how much road capacity to provide, but early methods tended to underestimate the growth. The methods were changed but then from the late 1980s systematically overestimated traffic growth, distorting the appraisal of benefits, and transforming the policy implications: it became evident that no feasible road capacity expansion would be enough to cope with the forecast traffic, and it would be necessary to manage demand instead. Since 2015 the official forecasts have sensibly avoided specifying a ‘most probable’ future, replacing it with a variety of different possibilities from almost no growth to exceedingly high. This creates a framework for a much more useful type of policy appraisal, though practical road proposals mostly still confidently assert high traffic growth at levels which have not been seen for over 25 years.