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

Transportations Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
different fuel sources (hydrogen and electricity) and therefore are discussed in Section 2.0,
Low-Carbon Fuels.
This report focuses on the potential GHG benefits of each of these vehicle technology
strategies if pursued individually. The benefit estimates rely on estimates of both pervehicle
GHG reductions, and the potential market penetration of a given vehicle type. As
such, the GHG benefits of individual vehicle technology strategies cannot be combined to
give total benefits. It is possible in the future that one particular technology path could be
taken, or that multiple technologies could be used in different applications, depending
upon where they are most advantageous. While beyond the scope of this report, estimates
of total GHG benefits achievable from the light-duty vehicle sector could be developed
through the analysis of alternative market penetration scenarios that combine different
technology types. Nevertheless, the information presented in this report is helpful in
assessing which vehicle technology options may provide the greatest benefits, as well as
the costs or cost savings that might be realized from each technology.
Trends in Light-Duty Vehicle Use and Efficiency
The total miles traveled by LDVs has increased quite steadily over the past 35 years, more
than doubling since 1970. During this time, both the total number of vehicles and the
average miles traveled per vehicle have increased considerably, as has the share of
heavier, lower fuel efficiency SUVs and light trucks, which now constituting about half of
all new vehicle sales in the United States (AEO Table 57). 8
All of these factors have
contributed to the steady growth in GHG emissions from LDVs. Other factors have
tended to diminish LDV emissions during this time, however. For example, the adoption
of the CAFE standards in the 1970s significantly constrained GHG reductions over time.
According to the 2008 EPA Fuel Economy Trends report, “Since 1975, the fuel economy of
the combined car and light truck fleet has moved through several phases: 1) a rapid
increase from 1975 to the early 1980s, 2) a slow increase to the fuel economy peak of
22.0 mpg in 1987, 3) a gradual decline from the peak to 19.3 mpg in 2004, and
4) consecutive annual increases beginning in 2005, growing to 20.8 mpg in 2008” (U.S.
EPA, 2008a). Figure 3.2 displays these trends since 1975.
While LDV fuel economy has remained relatively flat since the mid 1980s, the automotive
industry has continually improved the efficiency of the vehicles themselves. However,
these advances have been used to enhance vehicle performance and utility, and to permit
weight increases, rather than in the service of increasing miles per gallon ratings. In fact,
8 The sales fraction of light trucks and SUVs has been roughly constant since 2002 (U.S. EPA 2008),
and is assumed to remain at these levels into future years for the purposes of this analysis.
Heywood et al point out that even with sizable swings in these sales fractions (between 30 and
70 percent), the resulting impact on fleet wide GHG emissions varies only by about 2 percent
through 2035 (Heywood et al., 2008).
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