A newly published government-backed review has concluded that the long-established phenomenon of “induced demand” remains a significant factor in road investment decisions, finding that new road capacity often generates additional traffic that can erode congestion benefits over time.
The report, Impact of Induced Demand on Benefits, was commissioned by the Department for Transport and written by Professor Phil Goodwin, Emeritus Professor of Transport Policy, University College London and University of the West of England. It revisits evidence first highlighted by the Standing Advisory Committee on Trunk Road Assessment (SACTRA) more than 30 years ago and concludes that increasing road capacity to tackle congestion is likely to encourage additional vehicle trips, creating new delays and environmental impacts alongside the benefits delivered to road users.
The review examined how induced demand affects the appraisal of transport schemes and whether current modelling and assessment techniques adequately reflect the phenomenon. While recognising that additional capacity can deliver travel time savings and economic benefits, the report warns that these gains can be reduced when previously suppressed journeys, longer trips and route changes lead to higher traffic volumes.
Professor Goodwin says the evidence confirms that “the core problem established by SACTRA over 30 years ago still exists”, adding that building extra road capacity to solve congestion “is likely to induce extra road traffic”.
The report also highlights that induced demand is not limited to roads. Similar effects can occur when improvements are made to public transport, walking and cycling infrastructure, with travellers responding to changes in the perceived cost, convenience and attractiveness of different transport options.
The review stops short of arguing against road building, but suggests that failure to account fully for induced traffic risks overstating congestion relief and wider project benefits.
(File picture – Yay Images)
