Losing weight to save lives: A review of the role of automobile weight and size in traffic fatalities

Critics of higher fuel economy standards for vehicles have long argued that improving vehicle fuel economy will require reducing vehicle weight, and that would result in an increase in the number of fatalities from vehicle crashes. Several researchers have estimated that an across-the-board reduction of vehicle weight would reduce passenger safety (Evans 1991; Kahane 1997; U.S. GAO 1994). However, little research has been done on the relationship of vehicle size and fatality rates, independent of weight (see, however, Joksch, Massie, and Pichler 1998). In this report we review previous analyses of the relationship of vehicle weight and safety. We do this to study the opportunities to improve fuel economy in a more sophisticated way than across-the-board mass reduction. The aim is to explore improvements in traffic safety by making selected vehicle groups lighter, and retaining or enlarging selected vehicle dimensions. Unfortunately, the effects of size and mass have not been accurately separated in the crash fatality data, so some of our claims are only supported by general arguments from physics. In a follow-up report we will attempt to analyze crash fatality data to determine crash worthiness if the weight of certain vehicle groups would be decreased while maintaining or increasing vehicle size. The first section of this report provides an introduction to fatal vehicle crashes in the United States. We summarize several trends in automotive crash statistics over the last 20 years. Then we discuss approaches analysts have taken in studying factors that affect ''exposure'', the likelihood of being involved in a serious crash. In Section 2 we discuss standardized crash tests and the basic physics of crashes. We review previous research on the effect of vehicle mass in two-vehicle crashes in Section 3. We also present new research on reductions in risk achieved by recent light vehicles. Section 4 presents previous estimates of the effect of reductions in vehicle mass and size on the number of fatalities overall. Section 5 presents our scenario for making light duty vehicles safer while improving fuel economy.