Highway capacity won’t relieve congestion or lower emissions, studies conclude

By Eric Sunquist

A commonly cited strategy to achieve lower emissions and energy use is highway capacity expansion intended to reduce delay. Such a strategy, which has justified many a CMAQ project, is straightforward and appealing to motorists who dislike traffic.

But, as a new brief from UC-Davis and hosted on the Caltrans website points out, congestion relief is usually short-lived, due to “induced demand” or “induced travel.” Author Susan Handy writes:

“The basic economic principles of supply and demand explain this phenomenon: adding capacity decreases travel time, in effect lowering the ‘price’ of driving; and when prices go down, the quantity of driving goes up. Induced travel counteracts the effectiveness of capacity expansion as a strategy for alleviating traffic congestion and offsets in part or in whole reductions in GHG emissions that would result from reduced congestion.”

The brief, called “Increasing Highway Capacity Unlikely to Relieve Traffic Congestion,” describes seven research findings on the topic:

  • The quality of the evidence linking highway capacity expansion to increased VMT is high.
  • Increased roadway capacity induces additional VMT in the short run and even more VMT in the long run.
  • Capacity expansion leads to a net increase in VMT, not simply a shifting of VMT from one road to another.
  • Increases in GHG emissions attributable to capacity expansion are substantial.
  • Capacity expansion does not increase employment or other economic activity.
  • Conversely, reductions in roadway capacity tend to produce social and economic benefits without worsening traffic congestion.

The brief is relevant anywhere highway capacity is being considered. In terms of policy, it is particularly salient in California, where rulemaking that would require project sponsors to account for induced travel under the state’s Environmental Quality Act is under way.

Eric Sundquist is Managing Director of SSTI.