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More on this topic in TRAFFIC SAFETY   by Leonard Evans (published Aug 2004)
  CAFE and safety

May-June 2005


To the Editors:

Safety is ignored in "Fuel Efficiency and the Economy" by Roger Bezdek and Robert Wendling (March–April). CAFE regulations led to lighter vehicles, thereby increasing injury risk in traffic crashes (the subject of my own article in American Scientist, May–June 2003).

Due to physical laws, drivers of lighter cars in single-vehicle crashes experience higher decelerations, with consequently higher injury risks. Single-vehicle crashes produce half of driver deaths. While two-vehicle crashes involve more complex considerations, reducing the weight of one vehicle generally leads to a small increase in risk when averaged over both involved drivers. Nationwide, a lighter fleet kills more people. Details are given in my 2004 book Traffic Safety. The study cited by Bezdek and Wendling estimates that CAFE regulations increased annual deaths by 2,000.

By reducing driving costs, CAFE encourages more travel, less carpooling, and less use of alternative transportation. The average distance traveled by a vehicle in a year remained essentially constant at 10,000 miles between the end of World War II and the mid 1970s. Since CAFE started, it has increased to over 12,000 miles. Since CAFE started, the nation’s fuel use, and the percent of fuel that is imported, have both increased.

Leonard Evans
Bloomfield Hills, Michigan

Drs. Bezdek and  Wendling respond:

Significant fuel-efficiency improvements will require major changes in vehicle design, and safety is always an issue when vehicles are redesigned. However, the relation between fuel economy and highway safety is complex, poorly understood and difficult to measure. Improving vehicle fuel efficiency could be marginally harmful, beneficial, or have no impact on highway safety.

First, few of the technologies identified in our article require significant vehicle weight reductions, and to the extent that consumers value power and weight, manufacturers are reluctant to reduce either to improve fuel efficiency.

Second, it is the relative weight of vehicles rather than their absolute weight that leads to the adverse risk consequence for the occupants of the lighter vehicle, and there is evidence that proportionately reducing the mass of all vehicles would benefit safety in collisions.

Third, it is important to account for confounding factors and avoid drawing conclusions from spurious correlations. Because the driver is a far more important determinant of crash occurrences than the vehicle, even small confounding effects can lead to erroneous results. In his 1991 book Traffic Safety and the Driver, Dr. Evans indicates that the driver is one of the major factors in 94 percent of U.S. traffic crashes, the road environment in 34 percent of crashes, and vehicles in 12 percent. Moreover, younger drivers tend to drive smaller cars, smaller cars are more common in urban areas, older drivers are more likely to be killed in crashes of the same severity and so on. It is difficult to isolate the effects all of these factors and, in the case of vehicle weight and safety, adequate measures do not exist to isolate the effects of weight alone.

Dr. Evans is correct in stating that enhanced CAFE standards, by reducing the costs of driving, may encourage more travel. However, empirical studies have found that this effect is minor compared to the overall fuel savings from CAFE. That said, virtually all analysts agree that vehicle fuel efficiency standards are a "second best" alternative, and that substantially increasing gasoline taxes is the preferred policy. Unfortunately, such increased gasoline taxes are a political nonstarter in the U.S.
Sigma Xi, The Scientific Research Society