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

Transportation’s Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
friction. For this reason highway speed limit reductions have been considered as a means
of reducing fuel consumption across the country. The Emergency Highway Energy
Conservation Act of 1974 first instituted a nationwide speed limit of 55 mph with the goal
of reducing total on-road fuel consumption by two percent. However, the law was met
with widespread resistance and poor overall compliance, and was ultimately repealed in
1995. Since that time several States have raised their speed limits to pre-1974 levels, while
others have retained the lower speeds. An analysis of the 1974 speed limit concluded that
it did result in savings of about two percent of annual gasoline consumption while in
effect (TRB, 1984).
Speed limit reduction programs are again under consideration, largely in response to
climate change concerns, as well as rising gasoline and diesel prices. The economic value
of speed control has been recognized in the heavy-duty truck sector, with the American
Trucking Association (ATA) reporting that 77 percent of its members have adopted onboard
speed limiters restricting operation to 68 mph or less (Lavelle, 2008). In support of
climate action plans, States, including Arizona, Arkansas, California, Minnesota, New
Mexico, New York, and Utah also have conducted preliminary evaluations of the potential
fuel consumption and CO2 benefit associated with lowering average heavy-duty truck
speeds to 60 or 55 mph, lowering the maximum speed limit for all vehicles, and/or
through improved enforcement of speed limits. 24
Recent Congressional inquiries and
proposed legislation have prompted similar investigations for light-duty passenger
vehicles as well as heavy-duty vehicles (Coile, 2008).
The amount of fuel consumption benefits resulting from speed limit reductions will
depend largely on the effectiveness of enforcement. For example, noncompliance rates on
New York Interstates following the first national speed limit initiative were found to be
extremely high, with 83 percent of all vehicles traveling at speeds greater than 55 mph
(Coile, 2008). Another study also seems to indicate substantial tolerance on the part of law
enforcement, with only one percent of all speeding citations issued to vehicles traveling
less than 10 mph over the posted limit (Houston Chronicle, November 24, 2002).
Speed cameras are an available technology that can prove quite effective. Advanced
speed enforcement strategies such as Intelligent Speed Adaptation (ISA), which relies on
on-board determination of site-specific speed limits using GPS technology, may result in
improved compliance in the future. ISA systems may be implemented in varying degrees,
from simply notifying the driver of the speed limit violation to fully automatic governance
of vehicle speed. Even without considering driver acceptance, however, such systems
may be difficult to deploy in the near future due to high costs. For example, a recent ISA
pilot program in Sweden instrumented and maintained approximately 5,000 vehicles at a
cost of $12.7 million, or $2,500 per vehicle (FHWA, 2005b).
24 Maximum posted speeds for trucks range from 55 to 75 miles per hour, and are 5 to 15 miles per
hour lower than the maximum posted speed for cars in some States (FHWA, 2008e, Table 3-8).
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