Gasoline prices have increased greatly over the last several years. 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 legislated a national 55 mile per hour (mph) speed limit to reduce gasoline consumption in response to the 1973 Arab oil embargo. 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 In 1995, the 55 mph speed limit was repealed.
Recently, 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 mph, if a vehicle reduces its speed by 5 mph, its fuel economy can increase by about 5 to 10 percent, because of reduced air resistance (drag). Optimal speed for fuel economy for individual vehicles ranges widely, but is generally between 30 and 40 mph, depending on a vehicle's characteristics. 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.
Lowering speed limits reduces total fuel consumption. According to data relating to 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. Since the affect of speed on fuel consumption is exponential, extrapolating the fuel savings demonstrated in 1974, we can expect fuel consumption savings would be 20 to 30 percent if a national speed limit of 35 mph was implemented by Congress. According to DOE's 2008 estimate, a national speed limit of 35 mph could yield possible savings of 1,750,000 to 2,750,000 barrels of oil per day. With oil prices between $50 and $100 per barrel, this translates into a savings of $87,500,000 to $275,000,000 per day.
Our recommendation is that Congress immediately pass a national speed limit of 35 mph. Additional savings would occur if the speed limit was reduced to 15 mph.
References:
GAO-09-153R http://www.gao.gov/products/GAO-09-153R
