Relationships between Vehicle Size and Fatality Risk in Model Year 1985-93 Passenger Cars and Light Trucks

Charles J. Kahane, Ph.D.

Abstract

Fatality rates per million exposure years are computed by make, model and model year, based on the crash experience of model year 1985-93 passenger cars and light trucks (pickups, vans and sport utility vehicles) in the United States during calendar years 1989-93. Regression analyses calibrate the relationship between curb weight and the fatality rate, adjusting for the effects of driver age, sex and other confounding factors. The analyses estimate the change in fatalities (including occupants of the "case" vehicle, occupants of other vehicles in the crash, and pedestrians/bicyclists) per 100 pound weight reduction in cars or in light trucks. A 100-pound reduction in the average weight of passenger cars, with accompanying reductions (based on historical patterns) in other size parameters such as track width, and in the absence of any compensatory improvements in safety technology, is associated with an estimated increase of 302 fatalities per year (± 3-sigma confidence bounds range from an increase of 170 to an increase of 434). However, a 100-pound reduction in the average weight of light trucks is associated with an estimated decrease of 40 fatalities (± 3-sigma confidence bounds range from a decrease of 130 to an increase of 50). In car-light truck collisions, 80 percent of the fatalities are occupants of the cars. When light trucks are reduced in weight and size, they become less hazardous to occupants of passenger cars as well as pedestrians, bicyclists and motorcyclists. Conversely, growth in the weight and size of light trucks could increase hazards to those groups.

Executive Summary

Large vehicles have historically been more stable and provided more protection for their occupants than small ones, although those benefits to society might be offset if they present a greater hazard to other road users. Between 1975 and 1985, new passenger cars in the United States became twice as fuel-efficient, but their average curb weight dropped by nearly 1000 pounds, with corresponding reductions in other size parameters such as track width and wheelbase. During 1990-91, the National Highway Traffic Safety Administration (NHTSA) studied the safety effect of that weight and size reduction and concluded that it increased fatalities by nearly 2000 per year.

Between 1985 and 1993, the number of passenger cars on the road and their average weight remained quite stable, but the population of light trucks - pickup trucks, sport utility vehicles (SUV) and vans - increased by 50 percent, while the average weight of a new light truck increased by 340 pounds. By 1992, the number of fatalities in collisions between cars and light trucks exceeded the number in car-to-car collisions. In car-light truck collisions, 80 percent of the fatalities are occupants of the cars. That raises the question whether the growth in the number and weight of light trucks is having an adverse impact on the safety of passenger car occupants and other road users, possibly exceeding any safety benefits of the vehicle-weight increases for the occupants of the trucks.

The objective of this report is to estimate the relationship between curb weight and the fatality risk, per million vehicle exposure years, for model year 1985-93 passenger cars and light trucks, based on their crash experience in the United States from 1989 through 1993. "Fatality risk" includes all fatalities in the crash: not just the occupants of the "case" vehicle, but also the occupants of other motor vehicles, pedestrians, and bicyclists. In other words, the objective is to find the net effect on society, when vehicle weight is changed. Estimates are obtained for six fundamental crash types that, together, comprise most of the fatalities in the United States:

• Principal rollovers (not resulting from a collision)
• Collisions with objects (e.g., impacts with trees)
• Collisions with pedestrians, bicycles, or motorcycles
• Collisions with trucks over 10,000 pounds (Gross Vehicle Weight)
• Collisions with passenger cars
• Collisions with light trucks (pickups, SUVs, or vans)

The results for light trucks are new, while the findings for passenger cars are a completion and update of NHTSA's 1991 study. The principal reason for analyzing cars again is that NHTSA's 1991 analysis did not address three types of fatal collisions: those with pedestrians, big trucks and light trucks. Also, the safety environment has changed since the mid-1980's: more light trucks on the road, higher belt use, more female and older drivers. Because the analysis has been expanded to include all the major crash types, the results of this report supersede the 1991 findings for passenger cars. In view of the complexity and the high public interest in the issue of vehicle size and safety, a draft of this report was peer-reviewed by a panel of experts under the auspices of the Transportation Research Board of the National Academy of Sciences. The report was then revised in response to the panel's recommendations.

The analyses are based on accident data from the 1989-93 Fatal Accident Reporting System (FARS) and vehicle registration data from R.L. Polk's National Vehicle Population Profiles for 1989-93. Fatality rates per million exposure years (which include fatalities to occupants of all vehicles in the crash, plus any pedestrians) are computed by make, model and model year. Regression analyses calibrate the relationship between curb weight and the fatality rate, adjusting for the effects of driver age and sex, vehicle age, State, urban-rural, daytime-nighttime and other confounding factors. Information about the age of the "average" driver in each make-model, and many of the other control variables, is derived from 11 State accident files for 1989-93, based on crash involvements in which vehicles were standing still (waiting for traffic to clear or a green light) and got hit by somebody else. The regression analyses estimate the percentage increase or decrease in fatalities (including occupants of other vehicles and pedestrians) per 100 pound weight reduction in cars or in light trucks. The percentage changes are applied to the 1993 "baseline" fatalities to estimate the absolute effects.

The estimates indicate what might happen to fatalities if historical relationships are maintained between weight and other size parameters, such as track width, wheelbase, center-of-gravity height, and structural strength. The trends shown here are not necessarily what would happen if a specific vehicle were reduced only in weight while keeping all other vehicle characteristics the same or if there were radical changes in the materials or design of vehicles. Specifically, the effect of weight reductions on fatalities in passenger car rollovers might be smaller if weight could be reduced without changing track width. If all passenger cars on the road were reduced in weight by 100 pounds, while light trucks and other vehicles remained unchanged, and in the absence of any compensatory improvements in safety technology, the following effects on fatalities are estimated:

PASSENGER CARS: EFFECT OF 100 POUND WEIGHT REDUCTION
(light truck weights unchanged)

* nonsignificant

The effect of downsizing passenger cars would be a statistically significant increase of fatalities in rollovers, collisions with objects, big trucks, and above all, light trucks. The harm would be only slightly offset by a modest benefit for pedestrians, bicyclists and motorcyclists. The observed effect on fatalities in car-to-car collisions, if both cars in the collision were downsized, is not statistically significant. The largest relative increase, 4.58 percent, would be in rollovers, given the historical tendency that reduced mass means narrower, shorter, less stable cars. But the greatest absolute increase, 151 fatalities, would be in collisions between cars and light trucks, which were a much bigger safety problem in "baseline" 1993 (5,751 fatalities) than principal rollovers (1,754 fatalities).

Overall, a 100-pound reduction in the average weight of passenger cars, in the absence of any compensatory safety improvements, is estimated to result in 302 additional fatalities: a 1.13 percent increase over the baseline. This overall increase is statistically significant. Its 2-sigma confidence bounds range from 214 to 390. Two-sigma confidence bounds have been considered wide enough to include the likely range of error in past NHTSA evaluations. Given this evaluation's complex analysis approach, it might be appropriate to consider wider, 3-sigma confidence bounds. They range from 170 to 434. Either set of confidence bounds supports a conclusion that car weight reductions, given historical patterns of car design, would be associated with increases in fatalities. The current estimate is higher than NHTSA's 1991 study (approximately 200 lives per 100 pounds) because the 1991 study did not address collisions of cars with light trucks, big trucks and pedestrians.

If all light trucks on the road were reduced in weight by 100 pounds, while passenger cars and other vehicles remained unchanged, and in the absence of any compensatory improvements in safety technology, the following effects on fatalities are estimated:

LIGHT TRUCKS: EFFECT OF 100 POUND WEIGHT REDUCTION
(car weights unchanged)

* nonsignificant

Reducing the mass of light trucks would significantly increase the fatality risk of their occupants in collisions with objects and big trucks. But downsizing of light trucks would significantly reduce harm to pedestrians, motorcyclists and, above all, passenger car occupants. There would be little effect on rollovers because, historically, there has been little correlation between the mass of light trucks and their rollover stability (width relative to center-of-gravity height). There would also be little change in collisions between two light trucks, if both trucks are reduced in mass.

Even though the effect of mass reductions is statistically significant in four of the six types of crashes, the net effect for all types of crashes combined is small, because some of the individual effects are positive and others are negative. The benefits of truck downsizing for pedestrians and car occupants could more than offset the fatality increase for light truck occupants. It is estimated that a 100-pound reduction could result in a modest net benefit to society, a savings of 40 lives, (0.26 percent of baseline fatalities). However, this point estimate is not statistically significant: the 2-sigma confidence bounds range from a savings of 100 to an increase of 20 fatalities; the 3-sigma bounds range from a savings of 130 to an increase of 50 fatalities. It is concluded that a reduction in the weight of light trucks would have a negligible overall effect on safety, but if there is an effect, it is most likely a modest reduction of fatalities.

The results have a clear pattern: reducing a vehicle's weight increases net risk in collisions with substantially larger and stronger entities, reduces net risk in collisions with much smaller and more vulnerable entities, and has little effect on net risk in collisions with vehicles of about the same size. The only entities smaller than passenger cars are pedestrians, bicyclists and motorcyclists. Therefore, when car weight is reduced, the modest benefit for pedestrians is far outweighed by the increase in most other types of crashes. The latest light trucks, on the average, weigh over 900 pounds more than passenger cars. Continued growth in the number and weight of light trucks is likely to increase the hazard in collisions between the trucks and smaller road users (cars, motorcyclists, bicyclists and pedestrians), while a reduction in the weight of the trucks is likely to reduce harm in such collisions.

Some people believe that small cars attract aggressive drivers because they are more sporty and powerful than large cars. They might argue that, to a greater or lesser extent, it's not the cars, but rather their drivers that are responsible for the higher fatality rates of small cars in the preceding analyses. This belief may have been valid at one time, but today, the typical small car is no longer a sports car. The make-models currently associated with high performance, high horsepower, or aggressive driving are generally not small, but are of average or even slightly heavier-than-average weight. As a result, the high-performance make-models, if anything, biased the preceding analyses in favor of smaller cars. In a sensitivity test, the analyses of this report were re-run without those sporty and high-performance make-models. The correlation between passenger car weight and fatality risk did not diminish. In fact, it became slightly stronger. The predicted effect of a 100-pound weight reduction escalated from an increase of 302 fatalities in the baseline analysis to an increase of 370 fatalities on the sensitivity test.