Induced Travel Demand

Induced travel demand occurs when easier or cheaper road travel encourages people to travel more. Expanding urban road capacity can reduce journey times initially, but the resulting fall in the generalised cost of travel, including time and money, encourages additional vehicle use that can erode congestion relief.

How additional capacity attracts traffic

A wider road initially accommodates traffic more easily. However, travellers respond to the improved conditions rather than keeping their travel behaviour unchanged.

• Drivers shift from alternative routes, other travel times and public transport towards the improved road and peak period; this is called triple convergence.
• Lower travel costs also encourage additional trips, longer journeys and greater use of private vehicles.
• Traffic diverted from another route increases use of the expanded road but is not necessarily new travel across the urban network.

Why congestion relief weakens over time

As vehicle use increases, the initial speed advantage diminishes. Over longer periods, improved road accessibility can also influence where households and businesses locate.

• More dispersed development can lengthen journeys and strengthen car dependence, reinforcing demand for road space.
• Drivers generally do not bear the full congestion cost that their additional trips impose on other road users.
• Induced demand does not mean every road expansion is useless or that all benefits necessarily disappear; it means long-term relief cannot be estimated by assuming traffic demand remains fixed.

Implications for urban transport planning

Planning should prioritise accessibility, meaning access to jobs and services, rather than vehicle speed alone. Reliable public transport, safe walking and cycling, compact land use, parking management and congestion pricing can complement necessary road improvements by reducing dependence on additional road capacity.