The economic impact of obesity on automobile fuel consumption.

INTRODUCTION

Obesity has become an important public health issue in the United States. Since the early 1960s, the average adult weight has increased by more than 24 pounds (U.S. DHHS, 2004). In 2002, it was estimated that 65% of adults were overweight and over 30% of adults were obese, where overweight is defined as having a body mass index (BMI) of between 25 and 30, and obese is defined as having a BMI of over 30 (note that a BMI of 18.5-24.9 is considered normal) (Hedley et al., 2004). There are numerous health risks that have been associated with obesity. For example, obesity has been reported to increase the risk of coronary heart disease, hypertension, type-2 diabetes, stroke, and some types of cancer (U.S. DHHS, 2004, 2005). The health care costs to treat the complications of obesity were estimated to account for 9.1% of the total United States health care expenditures in 1998 (Finkelstein et al., 2003). Given that a growing number of people are forecasted to be classified as obese, these problems will remain important issues of concern to our nation's health care system (Hedley et al., 2004).

Beyond public health, obesity has other socio-economic implications. In particular, obesity increases the amount of fuel (i.e., gasoline) consumed by passenger vehicles (i.e., automobiles, including cars and light trucks driven for noncommercial purposes). In general, as the weight carried in vehicles increases, the amount of fuel consumed also increases. Note that this weight can be increased by other factors (e.g., excess cargo). The amount of fuel consumed by passenger vehicles can also be decreased, for example, by keeping vehicle engines appropriately tuned for optimal performance, correctly inflating tires, replacing clogged air filters, and accelerating less rapidly while driving (U.S. DOE and EPA, 2005). Note that most of the fuel consumed is to transport the vehicles themselves, which are generally much heavier than the loads being carried.

Steadily rising fuel costs have made fuel consumption a politically charged economic issue. In September 2005 (and again in April, July, and August 2006), the average cost of regular unleaded gasoline reached US$3 per gallon, its highest recorded level in the United States. Despite such historically soaring prices, fuel consumption remains strong (U.S. EIA, DOE, 2006). Of the approximately 20 million barrels of crude oil used in the United States each day, approximately 40% of it is consumed by passenger vehicles (Heavenrich, 2006). This has prompted several United States government agencies to promote the use of more fuel-efficient vehicles. They argue that this is a matter of national security, since reducing fuel consumption will simultaneously reduce dependency on foreign sources of oil (U.S. DOE and EPA, 2005). Reducing fuel consumption has other tangible economic and environmental benefits, including lower costs to drivers, decreased greenhouse gas pollution rates, and decreased air and noise pollution. However, the total amount of fuel consumed across all sectors has steadily risen since 2000 (U.S. DOT, BTS, 2006).

One way to reduce the amount of fuel consumed by passenger vehicles is to require automobile manufacturers to design more fuel-efficient vehicles. The amount of fuel consumed in the United States can be reduced over the course of several years as the fleet of passenger vehicles is gradually replaced by more fuel-efficient vehicles, assuming that driving habits remain the same. However, the total amount of fuel consumed nationwide may increase if the size of the passenger vehicle fleet grows or if vehicles travel greater distances (on average). In such cases, the increase in usage of fuel-efficient vehicles would only decrease the rate of growth in the total amount of fuel consumed.

Legislation that requires automobile manufacturers to produce fuel-efficient vehicles was first put forward in 1975, when the United States Congress enacted the Energy Policy Conservation Act (EPCA) to reduce foreign oil dependency following the 1973-1974 Arab oil embargo. The EPCA established the Corporate Average Fuel Economy (CAFE) standards for passenger vehicles (cars and light trucks) with gross vehicle weight ratings of less than 8500 pounds. The CAFE standards require that fuel economy (i.e., miles per gallon; mpg) of each automobile manufacturers' passenger vehicles--including wagons--average 27.5 mpg, and light trucks--including vans and sport utility vehicles (SUVs)--average 21.0 mpg. Model year 2007 light trucks are required to average 22.2 mpg, and model year 2011 light trucks are required to average 24.1 mpg (Heavenrich, 2006; U.S. DOT, NHTSA, 2006). The CAFE of a manufacturer's fleet of vehicles (weighing less than 8500 pounds) is computed as the sales weighted average of the vehicles' fuel economies, computed as the weighted average of the city (with weight .55) and highway (with weight .45) fuel economies. The fuel economy tests, which are identical for every vehicle, are performed in a controlled laboratory. The city fuel economy is determined over an 11-mile course with many stops and significant idling time, with the vehicle speed averaging approximately 20 miles per hour. The highway fuel economy is determined over a 10-mile course with the vehicle speed averaging approximately 48 miles per hour (U.S. DOE and EPA, 2004). Note that real-world fuel economy values actually observed are typically lower than these laboratory values.

The CAFE standards are determined by performing a cost-benefit analysis that weighs technological feasibility, economic practicality, the need to conserve energy, safety, and social costs (U.S. DOT, NHTSA, 2006). As a direct result of the CAFE standards, passenger vehicle fuel economy increased from (on average) 13.9 mpg in 1975 to 22.3 mpg in 2003, while light trucks increased from 10.5 mpg in 1975 to 17.7 mpg in 2003. Unfortunately, average fuel economy peaked in 1987, then declined until the mid-1990s, and has since remained relatively constant. This decline in fuel efficiency can be attributed to the popularity of SUVs, which are heavier and less fuel efficient than most cars, and which comprised more than 26% of new car sales in the United States in 2005 (Heavenrich, 2006). An indirect result of the CAFE standards is that the average weight of new cars and light trucks decreased each year from 1975 to 1987 (i.e., new vehicles with model years 1975-1987), though these weights have increased since 1988 (Heavenrich, 2006).

This article brings together the growing obesity epidemic and the need to reduce fuel consumption, by quantifying the amount of additional fuel consumed by passenger vehicles as a result of heavier passengers. The analysis considers passenger vehicles that are driven for noncommercial purposes. The key finding is that nearly one billion gallons of additional fuel per year can be attributed to the average weight gain between 1960 and 2002 of people living in the United States. This represents nearly three times the total amount of fuel consumed by all passenger vehicles each day based on current driving habits, or approximately 0.7% of the total amount of fuel consumed by passenger vehicles annually. Moreover, it is estimated that over 39 million gallons of fuel are consumed annually for every one pound increase in average passenger weight. Although the amount of fuel consumed due to the rising prevalence of obesity is small compared to the increase in the amount of fuel consumed due to other factors, such as increased vehicle usage and an overall increase in the number of drivers, in absolute terms, it still represents a large amount of fuel, and will become even more significant as the rate of obesity increases.

The analysis uses both historical and current weight data (collected between 1960 and 2002) to quantify the amount of fuel consumed due to increases in passenger weight. Current driving data collected in 2003 are used such that the amount of fuel consumed that is attributable to weight gains over the past four decades can be computed. Moreover, it is assumed that current driving habits remain constant regardless of changes in passenger weight. Note that the analysis does not make adjustments for differences in driving habits and socio-economic status. However, obesity is more prevalent among those in low-income and minority populations, who may be less likely to own a vehicle (Ogden et al., 2006; U.S. DHHS, 2000; U.S. DOT, BTS, 2003). This analysis also does not address safety issues. Obese people have been observed to be more likely to be killed or seriously injured as a direct result of being in car crashes (Zhu et al., 2006), with people with BMIs between 35 and 39 more than twice as likely to be killed in car crashes than people with BMIs of approximately 20 (Mock et al., 2002).

Motorcycles and recreational vehicles are not included in the analysis since they represent a negligible proportion of miles traveled and fuel consumed, and since there are few reliable data sources available for these modes of transportation. In this article, fuel refers to gasoline, and the amount of fuel consumed quantifies gallons of gasoline, rather than barrels of crude oil, consumed. Note that it is difficult to quantify the amount of crude oil consumed, since crude oil is converted to not only fuel, but to other petroleum products. Moreover, the number of gallons of fuel that can be refined from a barrel of crude oil varies with the quality of the crude oil. Note also that a similar analysis to the one reported in this article was performed for airline passengers. Dannenberg et al. (2004) note that an additional 350 million gallons of jet fuel was consumed in the United States in 2000 due to increases in the average American weight gain since 1990.

The article is organized as follows. The first section summarizes the data collected in the analysis. The following section introduces the models used to estimate the amount of fuel consumed due to changes in passenger weight and summarizes three cases to estimate total passenger weight in a vehicle. The next section performs an analysis of the amount of fuel consumed given historical weight data, compared to current weight data, followed by concluding comments and directions for future research.