Driving Conditions, Behaviors, and Traffic
Average fuel efficiencies of light-duty vehicles vary by 60% among industrialized countries. During the past 15 years, efficiencies have improved steadily in Western Europe but have remained relatively unchanged in Japan, Australia, and the United States.
Driving conditions and behaviors exert a major influence on average fuel efficiency. Newer vehicles have better aerodynamics and thus are more efficient at higher speeds than older vehicles. Nonetheless, due to wind resistance, (which is proportional to the square of a vehicle’s speed) vehicles moving at 75 mph (121 kmph) are still 20% less efficient than those moving at 55 mph (89 kmph). After the 1973 oil crisis, the U.S. Congress capped the speed limit on all roads at 55 mph to promote higher fuel efficiencies. This restriction was so unpopular that Congress raised the limit to 65 mph in 1987 and repealed the law entirely in 1995. Local authorities now set speed limits; the limit is 75 mph on many rural highways, and two remote stretches of road in Texas have a speed limit of 80 mph (129 kmph).
Speed, although important, is second to driving behavior—which is often a function of traffic–in determining fuel efficiency. Quick acceleration and heavy braking diminish fuel economy by more than 30%.  Worldwide, the number of vehicles and the distances they travel are many times greater than they were 2 decades ago, while the construction of new roads has lagged far behind. Such disparities have led to greater traffic congestion. The United States, for example, paved a large proportion of its roads during the past 50 years but extended the total length of roads by only 11%. Over this same period, the number of vehicles on these roads tripled. Not surprisingly, traffic congestion worsened. A 10-minute drive across a U.S. city during slack times took an average of 11 minutes during rush hours 20 years ago, but now takes nearly 13 minutes.
Roads, vehicles, and traffic in the United States; Length (millions of km) of paved, unpaved, or total roads in the United States; number of vehicles in the United States (millions); and ratio of travel time required during peak vs. slack hours in 437 U.S. cities of various sizes. After Bureau of Transportation Statistics 2002; Central Intelligence Agency 2007; Schrank and Lomax 2007.
Many strategies can relieve recurring traffic congestion. Constructing entirely new roads is an obvious but expensive approach. Municipalities usually find it more cost-effective to eliminate “choke points” (narrow passages) on existing roads by building new intersections or even new lanes between intersections. Even within the confines of existing roads and lanes, a number of other strategies may diminish recurring and nonrecurring traffic congestion. Implementing strategies to reduce traffic congestion not only improve fuel efficiencies, but advance public safety as well.
 Reed, P. and M. Hudson (2005) We Test the Tips: What Really Saves Gas? And How Much? edmunds.com, http://www.edmunds.com/advice/fueleconomy/articles/106842/article.html, accessed July 8, 2007.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
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