Induced traffic evidence undercuts the economic case for building more roads

Studies have shown that induced traffic – the additional traffic that arises from investment to increase road capacity – erodes the projected savings in travel time, writes David Metz

David Metz

16 December 2021

The re-assignment and redistribution of traffic after an increase in road capacity

David Metz

The usual case for increasing road capacity is it will relieve congestion. The intended outcome is that journeys are faster and easier. Yet this can lead to more frequent or longer car trips, changes to route or destination, or mode switching from public transport. All these changes lead to more traffic on the network.

The problem with induced traffic is that the more of it there is, the less the savings in travel time, which are treated as the main economic benefit of investment. So, the...