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entitled 'Energy Efficiency: Potential Fuel Savings Generated by a 
National Speed Limit Would Be Influenced by Many Other Factors' which 
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November 7, 2008: 

The Honorable John Warner: 

Ranking Member: 

Subcommittee on Private Sector and Consumer Solutions to Global Warming 
and Wildlife Protection: 

Committee on Environment and Public Works: 

United States Senate: 

Subject: Energy Efficiency: Potential Fuel Savings Generated by a 
National Speed Limit Would Be Influenced by Many Other Factors: 

Dear Senator Warner: 

Gasoline prices are volatile and have increased greatly over the last 
several years, before dropping again recently. The national average of 
regular grade retail gasoline prices increased from about $2.24 the 
week of January 2, 2006, to a peak of $4.11 the week of July 14, 2008, 
an increase of almost 84 percent, before dropping to about $2.40 the 
week of November 3, 2008 (see fig. 1). 

Figure 1: Weekly U.S. Retail Gasoline Prices, Regular Grade, January 
2006 through November 2008: 

This figure is a line graph showing weekly U.S. retail gasoline prices, 
regular grade, January 2006 through November 2008. The X axis 
represents the month and date, and the Y axis represents dollars per 

Jan. 2006: 223.8; 
Feb. 2006: 234.2; 
Mar. 2006: 233.1; 
Apr. 2006: 258.8; 
May 2006: 291.9; 
June 2006: 289.2; 
July 2006: 293.4; 
Aug. 2006: 303.8; 
Sept. 2006: 272.7; 
Oct. 2006: 231; 
Nov. 2006: 220; 
Dec. 2006: 229.7; 
Jan. 2007: 233.4; 
Feb. 2007: 219.1; 
Mar. 2007: 250.5; 
Apr. 2007: 270.7; 
May 2007: 305.4; 
June 2007: 315.7; 
July 2007: 295.9; 
Aug. 2007: 283.8; 
Sept. 2007: 279.6; 
Oct. 2007: 278.8; 
Nov. 2007: 301.3; 
Dec. 2007: 306.1; 
Jan. 2008: 310.9; 
Feb. 2008: 297.8; 
Mar. 2008: 316.2; 
Apr. 2008: 333.2; 
May 2008: 361.3; 
June 2008: 397.6; 
July 2008: 411.4; 
Aug. 2008: 388; 
Sept. 2008: 368; 
Oct. 2008: 348.4; 
Nov. 2008: 240. 

[See PDF for image] 

Source: Energy Information Administration. 

Note: Prices are in nominal terms and not adjusted for inflation. 

[End of figure] 

High fuel prices have focused attention on conservation. Congress 
previously used a national speed limit as an approach to conserve fuel 
when, in 1974, it provided for a national 55 mile per hour (mph) speed 
limit to reduce gasoline consumption in response to the 1973 Arab oil 
embargo. The law prohibited federal funding of certain highway projects 
in any state with a maximum speed limit in excess of 55 mph.[Footnote 
1] In 1987, Congress allowed states to raise the maximum speed limit to 
65 mph on rural interstate routes.[Footnote 2] In 1995, the 55 mph 
speed limit was repealed.[Footnote 3] Since then, states have been free 
to set speed limits without the loss of federal highway funds. 

You expressed interest in obtaining information on using a national 
speed limit to reduce fuel consumption. In response to your request, we 
reviewed existing literature and consulted knowledgeable stakeholders 
on the following: 

* What is the relationship between speed and the fuel economy of 

* How might reducing the speed limit affect fuel use? 

To address these two objectives, we relied on the expertise of GAO and 
knowledgeable stakeholders to identify the most relevant economic and 
transportation literature. Due to limited time and resources, we 
reviewed these studies and limited our analyses to light-duty vehicles, 
such as cars, sport utility vehicles, and pickup trucks. We identified 
the knowledgeable stakeholders from previous relevant GAO work. (For 
more details, see encl. I.) We provided the draft to the three agencies 
that we spoke to--the Environmental Protection Agency (EPA), the 
Department of Energy (DOE), and the Department of Transportation (DOT)-
-and incorporated relevant technical comments. We did not examine other 
aspects of implementing a national speed limit, such as potential 
safety impacts. 

In summary, according to these stakeholders and the relevant studies, 
reducing a vehicle's speed can potentially increase its fuel economy. 
However, the extent depends on a vehicle's characteristics, for 
example, its size and the efficiency of its engine and transmission. 
Furthermore, even though a lowered speed limit could reduce total fuel 
consumption, other factors--including driver behavior and road 
conditions and congestion--also affect fuel consumption. 

According to Literature and Stakeholders, Reducing a Vehicle's Speed 
Can Potentially Increase Its Fuel Economy, Depending on the Vehicle's 

For a vehicle traveling at high speed, reducing its speed increases 
fuel economy. In general, at speeds over approximately 35 to 45 mph, if 
a vehicle reduces its speed by 5 mph, its fuel economy can increase by 
about 5 to 10 percent, because air resistance, or drag, increases 
exponentially as a vehicle goes faster.[Footnote 4] Conversely, air 
resistance diminishes more rapidly as a vehicle slows down, thus 
increasing its fuel economy. 

According to existing literature and knowledgeable stakeholders, there 
is no single speed that optimizes fuel economy for all vehicles. 
Optimal speed for fuel economy for individual vehicles ranges widely, 
but is generally between 30 and 60 mph, depending on a vehicle's 
characteristics. For example, according to the most recent published 
data--a 1997 study by Oak Ridge National Laboratory, commissioned by 
the Federal Highway Administration (FHWA), that examined fuel economy 
at different speeds for nine automobiles and light trucks from model 
years 1988 through 1997--the optimal fuel economy for a 1994 Jeep Grand 
Cherokee, a sport-utility vehicle, would be about 26 miles per gallon 
at a steady 40 mph. In contrast, in a 2008 internal study by the 
Argonne National Laboratory for the Department of Energy (DOE), 
examining four vehicles, the optimal fuel economy for a 2005 Toyota 
Echo, a subcompact car, is about 69 miles per gallon, achieved when 
traveling at a steady 30 mph. Table 1 shows the speeds at which the 13 
vehicles included in those two studies achieve their optimal fuel 

Table 1: Optimal Speed for Fuel Economy for Studied Vehicles: 

Vehicle and year: 1997 Toyota Celica[A]; 
Optimal steady speed (mph) for fuel economy: 25; 
Miles per gallon: 52.6. 

Vehicle and year: 2005 Escape[B]; 
Optimal steady speed (mph) for fuel economy: 30; 
Miles per gallon: 32.4. 

Vehicle and year: 1993 Subaru Legacy[A]; 
Optimal steady speed (mph) for fuel economy: 30; 
Miles per gallon: 39.7. 

Vehicle and year: 2005 Echo[B]; 
Optimal steady speed (mph) for fuel economy: 30; 
Miles per gallon: 69.0. 

Vehicle and year: 1994 Jeep Grand Cherokee[A]; 
Optimal steady speed (mph) for fuel economy: 40; 
Miles per gallon: 25.5. 

Vehicle and year: 2005 Focus[B]; 
Optimal steady speed (mph) for fuel economy: 40; 
Miles per gallon: 45.1. 

Vehicle and year: 1994 Chevrolet Pickup[A]; 
Optimal steady speed (mph) for fuel economy: 45; 
Miles per gallon: 27.3. 

Vehicle and year: 1995 Geo Prizm[A]; 
Optimal steady speed (mph) for fuel economy: 45; 
Miles per gallon: 42.3. 

Vehicle and year: 1988 Chevrolet Corsica[A]; 
Optimal steady speed (mph) for fuel economy: 50; 
Miles per gallon: 31.2. 

Vehicle and year: 2005 Jaguar XJ8[B]; 
Optimal steady speed (mph) for fuel economy: 50; 
Miles per gallon: 37.8. 

Vehicle and year: 1994 Oldsmobile Cutlass[A]; 
Optimal steady speed (mph) for fuel economy: 55; 
Miles per gallon: 29.1. 

Vehicle and year: 1994 Mercury Villager[A]; 
Optimal steady speed (mph) for fuel economy: 55; 
Miles per gallon: 31.7. 

Vehicle and year: 1994 Oldsmobile Olds 88[A]; 
Optimal steady speed (mph) for fuel economy: 55; 
Miles per gallon: 34.6. 

Source: GAO analysis of DOE and FHWA data. 

Note: This table presents the most recently available data on speed and 
fuel economy for individual vehicles from two sources. 

[A] B.H. West, R.N. McGill, J.W. Hodgson, S.S. Sluder, D.E. Smith, 
Development and Verification of Light-Duty Modal Emissions and Fuel 
Consumption Values for Traffic Models. (Washington, D.C.: April 1997), 
and additional project data, April 1998. 

[B] Unpublished 2008 internal study by the Argonne National Laboratory 
for the Department of Energy (DOE). 

[End of table] 

However, a vehicle's fuel economy also depends on other factors besides 
air resistance. Factors that enhance fuel economy include engine 
efficiency enhancements (e.g., fuel injection), electronic and computer 
controls, more efficient transmissions, and hybrid technology. However, 
other factors decrease fuel economy (see fig. 2). 

Figure 2: Impact of Automotive Characteristics on Fuel Economy: 

This figure is a chart of impact of automotive characteristics on fuel 

Automotive characteristics: Engine efficiency (e.g., fuel injection); 
Impact on fuel economy: Increase. 

Automotive characteristics: Electronic and computer controls; 
Impact on fuel economy: Increase. 

Automotive characteristics: More efficient transmissions; 
Impact on fuel economy: Increase. 

Automotive characteristics: Hybrids; 
Impact on fuel economy: Increase. 

Automotive characteristics: Heavier vehicles; 
Impact on fuel economy: Decrease. 

Automotive characteristics: Bigger, more powerful engines; 
Impact on fuel economy: Decrease. 

Automotive characteristics: Increased accessory loads like air 
conditioning and electronics; 
Impact on fuel economy: Decrease. 

[See PDF for image] 

Source: GAO. 

Note: Impact on fuel economy assumes that all other factors are held 

[End of figure] 

In general, over the last 2 decades, fuel economy gains resulting from 
advances in automotive technologies have largely been offset by 
increases in vehicle weight, performance, and accessory loads. 
Specifically, vehicles are heavier than in the past, because they are 
larger and include more technologies. For example, average vehicle 
weight has increased from 3,220 pounds in 1987 to 4,117 in 2008, 
according to the Environmental Protection Agency (EPA).[Footnote 5] In 
addition, trends show that recent vehicles, on average, have bigger, 
more powerful engines that yield better performance--i.e., acceleration 
and greater speed--at the expense of fuel economy. For example, 
according to the same EPA report, average horsepower has increased from 
118 to 222 over the same period. Further, increased accessory loads, 
such as air conditioning and electronics, have also reduced fuel 
economy. According to EPA, from 1987 through 2004, on a fleetwide 
basis, technology innovation was utilized exclusively to support market-
driven attributes other than fuel economy, such as performance. 
Beginning in 2005, however, according to EPA's analysis of fuel economy 
trends, technology has been used to increase both performance and fuel 
economy, while keeping vehicle weight relatively constant. 

According to Literature and Stakeholders, a Reduced Speed Limit Is Only 
One of Many Factors That Could Affect Total Fuel Use: 

Lowering speed limits can potentially reduce total fuel consumption. 
According to literature we reviewed examining the impact of the 
national speed limit enacted in 1974, the estimated fuel savings 
resulting from the 55 mph national speed limit ranged from 0.2 to 3 
percent of annual gasoline consumption. According to DOE's 2008 
estimate, a national speed limit of 55 mph could yield possible savings 
of 175,000 to 275,000 barrels of oil per day.[Footnote 6] This range is 
consistent with estimates of the impact of the past national speed 
limit. According to the Energy Information Administration, total U.S. 
consumption of petroleum for 2007 was about 21 million barrels of oil 
per day. 

However, other factors, including drivers' compliance with a reduced 
speed limit, would affect the actual impact of a lower speed limit on 
the amount of fuel savings. Reducing the speed limit does not 
necessarily mean that drivers will comply. In fact, in 1975, under the 
previous national speed limit, about half of the states reported more 
drivers exceeding the national speed limit of 55 mph than complying 
with it. States may vary in their ability to enforce the reduced speed 
limit, in part due to cost and limited resources, affecting driver 

Moreover, a national speed limit would not affect many of the miles 
driven in the United States, such as those in urban areas, where most 
vehicles are already traveling at lower speeds due to lower speed 
limits or congestion. According to FHWA, fewer than one quarter of the 
vehicle miles traveled (VMT) in the United States would likely be 
directly affected by a changed speed limit. In addition, congestion 
forces some vehicles to travel slowly, no matter what the speed limit, 
meaning a reduction would have little or no impact on fuel consumed on 
congested roads. 

Other external conditions also affect fuel economy, such as road 
conditions, including whether a road is steep or flat, and weather 
conditions, including wind speed and direction. Finally, other aspects 
of driver behavior may also affect fuel consumption. For example, 
driver behavior may be affected by fuel prices. Higher prices may cause 
people to drive less or purchase more fuel-efficient vehicles. 
Similarly, driving at a consistent speed can reduce fuel consumption. 
In contrast, aggressive driving such as accelerating or stopping 
quickly can increase fuel consumption. In addition, proper vehicle 
maintenance--including regularly changing automobile fluids and filters 
and properly inflating tires--improves fuel economy. 

The speed limit is only one tool among many for potentially conserving 
fuel. Certain realities, such as congestion on our nation's roads, how 
people drive and maintain their vehicles, and emerging technologies, 
are other potential considerations as the nation looks for options to 
conserve fuel. 

We are sending copies of this report to interested congressional 
committees, the Environmental Protection Agency Administrator, the 
Secretary of Energy and the Secretary of the Department of 
Transportation. We also will make copies available to others on 
request. In addition, this report will be available at no charge on the 
GAO Web site at [hyperlink,]. 

Sincerely yours, 

Signed by: 

Mark E. Gaffigan: 

Director, Natural Resources and Environment: 

Signed by: 

Susan A. Fleming: 

Director, Physical Infrastructure: 


[End of section] 

Enclosure I: Scope and Methodology: 

To identify literature pertaining to these two objectives, we relied on 
the expertise of GAO to conduct an initial search of economic and 
transportation literature. Through this search, we identified two 
seminal studies that summarized the relevant research through 1997. We 
subsequently conducted a search for literature published after 1997 in 
databases such as National Technical Information Service (NTIS), 
Transportation Research Information Services (TRIS), and EconLit using 
key words such as "speed limit" and "energy use." This search 
identified additional selected reports and studies issued by federal 
and state agencies, transportation and energy research organizations, 
and academia. We also interviewed officials from Department of 
Transportation, Department of Energy, and Environmental Protection 
Agency as well as representatives from various associations with 
relevant experience and knowledge and asked them to identify recent 
literature that may pertain to our two objectives. We identified the 
knowledgeable stakeholders from previous relevant GAO work. 

Using these methods, we identified studies that were mentioned in both 
the literature and by the knowledgeable stakeholders. However, due to 
limited time and resources, we were only able to review a limited 
number of studies. As such, we identified a set of key studies from our 
list of relevant research to include in our review. We selected these 
studies judgmentally based on (1) relevance to the current work and (2) 
soundness of the methodology. A GAO economist and a technologist 
reviewed the methodology and scientific reasoning of these selected 
literature and found them to be sound and sufficiently reliable for the 
purposes of this report. Also, we limited our analyses to light-duty 
vehicles, such as cars, sport utility vehicles, and pickup trucks. We 
did not examine other aspects of implementing a national speed limit, 
such as potential safety impacts. 

[End of section] 

Enclosure II: GAO Contacts and Staff Acknowledgments: 

GAO Contact: 

Mark Gaffigan (202) 512-3841 or and Susan Fleming 
(202) 512-2834 or 

Staff Acknowledgments: 

In addition to the contacts named above, key contributors to this 
report were Karla Springer and Raymond Sendejas, Assistant Directors; 
Cindy Gilbert; Terence C. Lam; Tina Y. Paek; Madhav S. Panwar; Amy 
Rosewarne; Ilga Semeiks; Joseph D. Thompson; and Barbara Timmerman. 

[End of section] 


[1] Pub. L. No. 93-239, 2, 87 Stat. 1046-1047 (1974). The prohibition 
on funding was extended indefinitely in 1975. Pub. L. No. 93-643, 154, 
88 Stat. 2281, 2286 (1975). 

[2] Pub. L. No. 100-17, 174, 101 Stat. 132, 218 (1987). 

[3] Pub. L. No. 104-59, 205(d), 109 Stat. 568, 577 (1995). 

[4] National Research Council, Committee for Study of Impacts of 
Highway Capacity Improvement on Air Quality and Energy Consumption, 
Special Report 245, Expanding Metropolitan Highways: Implications for 
Air Quality and Energy Use (Washington, D.C.: National Academy Press, 
1995), p. 63. 

[5] Light-Duty Automotive Technology and Fuel Economy Trends: 1975 
through 2008 (Washington, D.C.: U.S. Environmental Protection Agency, 
September 2008) 

[6] In calculating these estimates, DOE assumed, among other things, a 
compliance rate of 50 percent and that the speed limit would affect 35 
percent of on-road (highway) mileage, which means roughly a third of 
travel is on roads where a decrease in the speed limit would have an 
effect. DOE's estimates include savings from on-road heavy duty trucks.

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