Energy Efficiency:

Potential Fuel Savings Generated by a National Speed Limit Would Be Influenced by Many Other Factors

GAO-09-153R: Published: Nov 7, 2008. Publicly Released: Nov 7, 2008.

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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. 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. In 1987, Congress allowed states to raise the maximum speed limit to 65 mph on rural interstate routes. In 1995, the 55 mph speed limit was repealed. Since then, states have been free to set speed limits without the loss of federal highway funds. Congress expressed interest in obtaining information on using a national speed limit to reduce fuel consumption. In response to the request, we reviewed existing literature and consulted knowledgeable stakeholders on the following: (1) What is the relationship between speed and the fuel economy of vehicles? (2) How might reducing the speed limit affect fuel use?

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. 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. 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. 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. 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. 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. 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. 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. 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. 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.

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