Induced travel demand: an evidence review

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Literature review on the evidence for induced travel demand, the increased vehicle traffic occurring from road capacity improvements.

Department for Transport (2018), Latest evidence on induced travel demand: an evidence review.

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Induced demand for road travel can be broadly defined as ‘the increment in new vehicle traffic that would not have occurred without the improvement of the network capacity’. […] unless induced traffic is correctly taken account of, significant errors in benefit estimation can be made.

There remain wide variations in the quantitative evidence that make it difficult to draw conclusions about the magnitude of the impact of induced demand from road capacity improvements on the Strategic Road Network. However, we draw some tentative conclusions:

  • Findings for state level road networks in the US and the national Dutch network indicate an elasticity of around 0.2 across the whole road network, i.e. a 10% increase in road capacity could lead to 2% induced demand on the network.
  • Induced demand is likely to be higher for capacity improvements in urban areas or on highly congested routes.
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There is a concept that goes hand-in-hand with induced demand that transportation professionals rarely acknowledge exists, let alone act upon. It is called reduced demand. If road capacity is reduced, people will also change their behavior.

Walkable City also mentions reduced demand, which is “what happens when when ‘vital’ arteries are removed from cities. The traffic just goes away,” Speck says. People may stop making certain trips, condense multiple trips into one, use alternative transportation options like walking, biking, or transit, or travel at different times of the day.

Reduced demand explains why “carmageddon” never occurred in numerous historical instances where traffic capacity was suddenly eliminated. For example, a 60-foot section of the West Side Highway in Manhattan collapsed in 1973. The highway carried 80,000 vehicles a day. A city traffic engineer at the time, Sam Schwartz, was tasked with measuring the impact on the vehicles on nearby city streets. To his amazement, about half of the traffic could not be found at all on nearby streets, and the rest was absorbed without major impact on the city’s grid.

People made personal transportation choices to adjust to the new reality. The highway was never rebuilt—instead a surface boulevard was constructed, with much better results for the city.

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