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

Transportation’s Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
Magnitude and Timing of Greenhouse Gas Reductions
Optimal operating speeds vary depending on a number of factors. An evaluation of 11
different light-duty gasoline makes and models found their optimal speed ranged from 25
to 55 mph (GAO, 2008). Accordingly, the benefit of establishing a uniform speed limit will
vary from vehicle to vehicle. In addition, the fuel consumption benefit also varies
depending upon the initial baseline speed of the vehicle. For example, using EPA’s
Physical Emission Rate Estimator (PERE) model, a 2009 Toyota Camry is estimated to
improve its fuel efficiency by approximately 2.4 mpg when slowing from 60 to 55 mph. A
larger reduction from 70 to 55 mph will result in a 6.1 mpg improvement (U.S. EPA,
2005a). Heavy-duty diesel vehicles are estimated to obtain a 0.1 mpg improvement
(approximately 1.5 percent improvement) on average per one mph of speed reduction at
highway speeds (U.S. EPA, 2004).
Special Report 204: 55: A Decade of Experience, published in 1984 by the Transportation
Research Board, evaluated the national 55 miles per hour speed limit imposed in 1974 and
concluded that it resulted in savings of about two percent of annual gasoline consumption
while in effect (TRB, 1984).
A recent DOE evaluation estimated that a 55 mph speed limit implemented at the national
level could result in a fuel consumption savings between 175,000 and 275,000 barrels of oil
per day, or about 27 to 43 mmt CO2e per year, which represents about 1.6 to 2.4 percent of
on-road vehicle fuel consumption and emissions. For this assessment DOE assumed that
35 percent of all on-road mileage would be impacted by the reduced speed limit, along
with a 50 percent compliance rate (GAO, 2008). The non-compliance rate ranged from
49% to 54% of traffic exceeding 55 mph on all highways marked at 55 mph for the period
1980 to 1983, according to speed data reported by the States to the Federal government
from automated devices embedded in pavements (TRB, 1984). Future compliance rates
would depend on enforcement and public acceptance.
An International Energy Agency study using a mechanical engineering equation of fuel
efficiency by speed found that comprehensive enforcement of a 55 mph speed limit could
generate a reduction of up to 3.2 percent in fuel consumption and thus GHG emissions in
the U.S. (IEA, 2005), or about 56 mmt CO2e per year.
The benefits will be lower if vehicle fuel efficiency improves beyond the baseline levels
assumed in this analysis. Changes in vehicle technology also could affect the magnitude
of benefits from speed limit reductions. For example, hybrids are expected to constitute
an increasing fraction of the light-duty fleet over time. Fuel consumption in light-duty
hybrid gasoline vehicles rises disproportionately quickly above 50 mph, compared with
conventional vehicles. To illustrate, the PERE model predicts a 9.5 mpg improvement in
fuel economy for hybrid vehicles slowing from 70 to 55 mph, compared to just 6.1 mpg for
comparable conventional gasoline vehicles. The higher baseline fuel economy makes total
fuel consumption impacts lower in absolute terms, however. A European study found
that GHG emissions from modern vehicles tend to show much less dependence on vehicle
speeds (at least at highway speeds) compared with older vehicles (Carsten et al., 2008).
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