Explaining Induced Traffic

Eric Jaffe at The Infrastructurist explains the non-intuitive reason why often removing freeways means less traffic.

Jaffe turns to a study by two economists from the University of Toronto, Gilles Duranton and Matthew Turner:

"Duranton and Turner analyzed loads of data on traffic, infrastructure, and travel behavior from metropolitan regions across the United States and found that 'vehicle-kilometers traveled increases proportionately to roadway lane kilometers for interstate highways.' For those who don't care for either academic abstracts or the metric system, the authors then parse their conclusion in pithier terms: 'roads cause traffic.' The basis for this confusing reality, write Duranton and Turner, is a three-pronged 'fundamental law of highway congestion' that explains why road construction can never keep pace with road congestion..."

Full Story: Why Building Roads Creates Traffic



Todd Litman's picture

Induced Traffic: Implications for Transport Planning

People interested in this issue may want to read my article, “Generated Traffic; Implications for Transport Planning,” published in the April 2001 issue of the ‘ITE Journal.’ An updated version is available free at www.vtpi.org/gentraf.pdf .

This was the first article concerning generated traffic and induced travel published in a practitioners’ journal (as opposed to journals for transport economists and modelers), and included not just a review of evidence of induced travel, but also discussion of the implications for transport project evaluation. There are three:

1. Induced travel reduces predicted congestion reduction benefits. Traffic
models that fail to fully account for generated traffic exaggerate roadway
expansion benefits and the severity of future congestion problems if roadways
are not expanded (congestion actually tends to maintain self-limiting equilibrium).

2. Induced travel consists of marginal value vehicle travel, that is, the mileage
motorists consider to have least net user benefits and so are most willing to
forego if their generalized costs (time and money) increases. As a result, the
additional VMT provides only modest user benefit (consumer surplus).

3. Induced travel increases external costs, including downstream congestion
(for example, if highway expansion increases surface street traffic volumes),
road and parking facility costs, accidents, energy consumption, pollution
emissions, and sprawl-related costs.

This is not to say that roadway expansion and increased vehicle travel provide no benefits, but failing to account for these three factors tend to significantly overstate the net long-run benefits of roadway expansion, and therefore undervalues alternative congestion reduction strategies such as pricing reforms, improvements to alternative modes, or more accessible land use development which reduces travel distances.

Duranton and Turner found no reductions in congestion from public transit investments because they measured total transit (buses and trains together) and total VMT (peak and off-peak), but other studies which use different methodologies (analyzing individual corridors) do indicate that high quality, grade separated public transit reduces congestion delays on parallel roadways.

For more discussion of these issues see:

“Smart Congestion Reductions: Reevaluating The Role Of Highway Expansion For Improving Urban Transportation,” (www.vtpi.org/cong_relief.pdf ).

“Evaluating Rail Transit Benefits: A Comment,” Transport Policy, Vol. 14, No. 1, 2007, pp. 94-97; an updated version in “Smart Congestion Reductions II: Reevaluating The Role Of Public Transit For Improving Urban Transportation” (www.vtpi.org/cong_reliefII.pdf ).

Todd Alexander Litman
Victoria Transport Policy Institute
"Efficiency - Equity - Clarity"

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