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

U.S. Department of Transportation http://www.nhtsa.dot.gov National Highway Traffic Safety Administration DOT HS 808 570 January 1997 NHTSA Technical Report Relationships between Vehicle Size and Fatality Risk in Model Year 1985-93 Passenger Cars and Light Trucks This document is available to the public from the National Technical Information Service, Springfield, Virginia 22161. The United States Government does not endorse products or manufacturers. Trade or manufacturers' names appear only because they are considered essential to the object of this report. Technical Report Documentation Page 1. Report No. 2. Go ,i on No. 3, Recipient's Catalog No. DOT HS 808 570 4. Title ond Subtitle 5. Report Dote January 1997 Relationships Between Vehicle Size and Fatality Risk 6. Performing Organization Code in Model Year 1985-93 Passenger Cars and Light Trucks 8. Performing Organization Report No 7. Author's) Charles J. Kahane, Ph.D. 9. Performing Organization Name ond Address 10. Wort Unit No. (TRAIS) Evaluation Division, Plans and Policy National Highway Traffic Safety Administration 11. Conrroct or Grant No. Washington, D.C. 20590 13. Type of Report and Period Cohered 12. Sponsoring Agency Name and Address Department of Transportation NHTSA Technical Report National Highway Traffic Safety Administration Sponsoring Agency Code Washington, D.C. 20590 15. Supplementary. Notes NHTSA Reports DOT HS 808 569 through DOT HS 808 575 address vehicle size and safety. 16. 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. 17. Key Words 18. Distribution Stotement mass; weight; vehicle size; fatal crash; Document is available to the public through fatality rates; FARS; regression; the National Technical Information Service, fatality analysis; statistical analysis; . Springfield, Virginia 22161 evaluation; small cars; light trucks 19. Security Claisif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Pri-ee Unclassified Unclassified 263 Form DOT F 1700.7 (8-72) Reproduction of completed page authorized TABLE OF CONTENTS Executive summary v 1. Objectives, background, analysis methods and data sources 1 1.1 NHTSA's need to study size-safety relationships 1 1.2 How vehicle size can affect safety 2 1.3 Factors that confound size-safety analyses 4 1.4 Analysis strategies and data sources 7 1.5 NHTSA's earlier size-safety studies 10 2. Fatalities per 1000 induced-exposure crashes in 11 States: data 15 2.1 Analysis objective 15 2.2 Vehicle classification and specifications 15 2.3 EPA's weight measurements: trends and comparisons 18 2.4 State data reduction 20 2.5 FARS data reduction 22 2.6 Unadjusted fatality rates per 1000 induced-exposure crashes 27 3. Fatalities per 1000 induced-exposure crashes in 11 States: results 37 3.1 Logistic regression: setting up the variables 37 3.2 Revision of the induced-exposure databases 39 3.3 An example: logistic regression of passenger car rollovers 41 3.4 Regressions on the size of the "case" passenger car 44 3.5 Regressions on the size of the "case" light truck 49 3.6 Regressions on the weight of both vehicles: car-to-car 54 3.7 Regressions on the weight of both vehicles: truck-to-truck 59 3.8 Regressions on the weight of both vehicles: car-to-truck 59 4. Induced-exposure crashes per 1000 vehicle years in 11 States 63 4.1 Analysis objective 63 4.2 Polk data reduction 63 4.3 Unadjusted accident rates per 1000 vehicle years 65 4.4 Regression analyses ,71 4.5 Comparison with National Personal Transportation Survey data 80 4.6 Sensitivity tests 85 in 5. Fatalities per million vehicle years in the United States: analysis methods 89 5.1 Analysis objective 89 5.2 Data reduction 89 5.3 Unadjusted fatality rates per million vehicle years 92 5.4 Initial regressions of passenger car rollovers 112 5.5 Exogenous coefficients for driver age and sex 118 5.6 Discussion of the vehicle-weight and driver-age coefficients 119 6. Fatalities per million vehicle years in the United States: findings and sensitivity tests .... 123 6.1 Regressions on the size of the "case" passenger car 123 6.2 Regressions on the size of the "case" light truck 130 6.3 Effect of weight reductions on the number of fatalities 136 6.4 Sensitivity tests on the coefficients for driver age and gender 143 6.5 Sensitivity tests: exclusion of high-performance and sporty vehicles 147 6.6 Linearity of the weight-safety relationships 154 6.7 Sensitivity tests: concentrating the weight reductions on the heaviest vehicles .... 165 References 173 Appendix A Valid VIN1-VIN3 combinations for 1981-93 vehicles 177 Appendix B Fundamental car groups, 1985-93 187 Appendix C Fundamental light truck groups, 1985-93 215 Appendix D Curb weight, track width and wheelbase of passenger cars 239 Appendix E Curb weight, track width and wheelbase of light trucks 251 Appendix F Summary and response to TRB's recommendations on the draft report 261 IV 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) Fatalities Effect of Net in 1993 100 Pound Fatality Crash Type Crashes Weight Reduction Change Principal rollover 1754 + 4.58% + 80 Hit object 7456 + 1.12% + 84 Hit ped/bike/motorcycle 4206 - .46% - 19 Hit big truck 2648 + 1.40% + 37 Hit another car 5025 .62% (nonsignificant) - 31 Hit light truck 5751 + 2.63% + 151 OVERALL 26840 + 1.13% + 302 ±2-sigma confidence bounds +214 to +390 ±3-sigma confidence bounds +170 to +434 VI 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.

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