Transportation's Role in Reducing U.S. Greenhouse Gas Emissions ...

Transportation’s Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
(VMT) will result. The induced VMT is likely to come partly from shifts of
travelers from other modes (particularly transit) and partly from changes in
travel patterns (more trips and longer trips). Also, in some cases, there may be
shifts in the time of travel from off-peak periods to peak periods.
There are two basic types of transportation GHG reduction measures that can
result in induced demand:
• System efficiency improvements that reduce congestion and delay, thereby
improving travel times (as well as reducing fuel consumption and GHG
related to delay); and
• Travel behavior strategies that reduce VMT by increasing the attractiveness
of alternatives to auto travel (or single-occupancy-vehicle travel).
For example, policies that cause diversion to transit reduce highway congestion
and thereby reduce highway travel times, making highway travel more
attractive to non-transit users who, in turn, increase somewhat the number
and/or length of their highway trips. Travel behavior strategies will only result
in induced demand to the extent that they reduce VMT during congested travel
periods, and therefore reducing delay and decreasing travel times. This is
sometimes referred to as a “rebound effect,” since traffic volume rebounds as
the traffic that is diverted off the road is partially replaced by new traffic from
other sources. (Strategies that reduce VMT by making highway travel more
expensive—such as mileage-based fees or increased gas taxes—do not produce
a rebound effect, since they apply comprehensively to all highway travel.)
The offsetting effects of induced demand apply to any VMT or
congestion/delay-related metric such as fuel consumption or criteria pollutant
emissions. The magnitude of these effects depends upon the elasticity of travel
demand with respect to a change in travel time or travel cost—i.e., the percent
change in travel for a given percent change in time/cost. Both short-term
(about one year) and long-term (multi-year) elasticities have been estimated,
since rebound effects can be greater over the long term as people make more
significant changes to their travel habits such as living farther from work.
The magnitude of the induced demand effect from both system efficiency and
travel activity strategies was estimated in work performed for the Moving Cooler
study. The effects are quite different:
• For travel activity strategies, the system-wide “rebound effect” was
estimated to be about 14 percent, using the Federal Highway
Administration’s (FHWA) Highway Economic Requirements System
(HERS) model. That is, for any measure that would reduce VMT by making
alternatives to auto travel (or single occupancy vehicle travel) more
attractive, the initially estimated reduction in VMT and GHG is reduced by
14 percent to reflect the rebound effect. This reduction is reflected in the
travel activity strategy results cited from the Moving Cooler study for transit,
land use, non-motorized travel, and employer trip reduction.
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