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

Transportation's Role in Reducing U.S. Greenhouse Gas Emissions: Volume 1
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inherent uncertainties. For example, some of the vehicle efficiency and lowcarbon
fuel technologies that could yield the greatest long-term benefits are still
unproven. There is also no guarantee that these technologies can be advanced on
a large scale to the point of feasibility and cost-competitiveness. Future fuel costs
(as well as the relative difference in costs between fuels) will have a large impact
on the cost-effectiveness of particular fuel-saving and alternative-fuel strategies,
thereby affecting their ability to penetrate the market. For example, during
periods of high petroleum prices, alternative fuels may be cheaper than gasoline,
and vice versa. Therefore, both the magnitude and timing of these strategies
should be considered uncertain.
In addition to technological uncertainty, many of the strategies from all
categories may face significant political and/or institutional barriers. For
example, some of these strategies may require greater up-front vehicle purchase
costs (even if yielding net lifetime cost savings); significant public-sector
investment; and may create negative impacts on some populations (even if
yielding net social benefits). Carefully-crafted policies should seek to minimize
any negative impacts of these strategies, and maximize the social benefits that are
achieved beyond GHG reductions. Policy options for implementing GHG
reduction strategies are discussed in Volume 1, Section 5.0 of this report.
For many strategies there is limited empirical evidence available to base findings.
Often, only one or two studies have examined the GHG benefits of a particular
strategy. A number of strategies had aspects that could not be evaluated due to
this lack of empirical evidence. Even where multiple studies exist, they
sometimes indicate a potentially wide range of benefits, reflecting uncertainty
regarding the benefits or costs of these strategies. Professional judgment was
applied in selecting the most comprehensive and reliable assessments to draw
from for each strategy. In Volume 2, the level of confidence in the estimates, and
any particularly important sources of uncertainty, are discussed for each strategy
assessed.
Finally, there is uncertainty regarding consumer response to changes in travel
conditions. Strategies that smooth traffic flow by reducing stop and go
congestion may reduce emissions, but the improved travel conditions can also
lead to increased travel, offsetting emissions benefits. While this concept, called
induced demand, is widely acknowledged in the transportation profession,
estimates of its magnitude are a source of uncertainty and debate. A range of
plausible estimates from the literature could significantly impact induced
demand and GHG calculations for many strategies. This study cites analysis that
incorporates the impact of induced demand for strategies that would improve
travel conditions–highway operations and management, public transportation,
and commute travel reduction—with the exception of aviation, rail, and marine
operations, where insufficient data was available.
For most strategies, a range of potential benefits is shown, reflecting some of the
uncertainties inherent in the assessment. The ability to achieve even the lower
end of the range, however, is by no means guaranteed. Therefore, the results