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

© U.S. Department http://www.nht8a.dot.gov of Transportation National Highway Traffic Safety Administration January 1997 DOT HS 808 570 Janua y 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 tha 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 ofthis report. Technical Report Documentation Page I. Report No. 2. Government Accession No. 3. Recipient s Catalog No. DOT HS 808 570 4. Title and Subtitle 5. Report Date January 1997 Relationships Between Vehicle Size and Fatality Risk 6. Performing Organisation Code in Model Year 1985-93 Passenger Cars and LightTrucks 8. Performing Organisation Report No. 7. Author's! Charles J. Kahane, Ph.D. 9. Performing Organisation Name and Addreit 10. Work Unit No. (TRAISI Evaluation Division, Plans and Policy National Highway Traffic Safety Administration II. Contract or Grant No. Washington, D.C. 20590 13. Type of Report ona Period Covered 12. Sponsoring Agency Nome and Address Department ofTransportation NHTSA Technical Report National Highway Traffic Safety Administration Id. 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 bymake, model and model year, based onthecrash experience of model year 1985-93 passenger cars and light trucks (pickupSj 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 ofthe "case" vehicle, occupants of other vehicles in thecrash, 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 onhistorical patterns) inother size parameters ' such as track width, and inthe absence ofany compensatory improvements in safety technology, is associated with anestimated increase of302 fatalities per year (±3-sigma confidence bounds range from an increase of 170 to an increase of434). However, a 100-pound reduction inthe average weight oflight trucks is associated with an estimated decrease of40 fatalities (± 3-sigma confidence bounds range from adecrease of 130 to an increase of50). In car-light truck collisions, 80 percent ofthe 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 intheweight and size oflight trucks could increase hazards to those groups. 17. Key Words 18. Distribution Statement mass; weight; vehiclesize; 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 dossil, (of this report) 30. Security Classif. (pf this page) 21- No. of Poges 22. Puce Unclassified Unclassified 263 Form DOT F 1700.7 (8-72) Reproduction of completed page authorized to-" 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 *5 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 upthe variables 37 3.2 Revision oftheinduced-exposure data bases 39 3.3 Anexample: logistic regression of passenger car rollovers 41 3.4 Regressions onthe size ofthe"case" passenger car 44 3.5 Regressions onthe size ofthe "case" light truck 49 3.6 Regressions ontheweight ofboth vehicles: car-to-car 54 3.7 Regressions ontheweight ofboth vehicles: truck-to-truck 59 3.8 Regressions ontheweight ofboth 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 89g9 5.2 Datareduction g_ 5.3 Unadjusted fatality rates per million vehicle years ~ 5.4 Initial regressions of passenger carrollovers jJj 5.5 Exogenous coefficients for driver age and sex jjJ 5.6 Discussion ofthe vehicle-weight and driver-age coefficients ny 6. Fatalities per million vehicle years in the United States: findings and sensitivity tests .... 123 6.1 Regressions on the size ofthe "case" passenger car J23 6.2 Regressions on the size ofthe "case" light truck J30 6.3 Effect ofweight reductions on the number offatalities 136 6.4 Sensitivity tests on the coefficients for driver age and gender 143 6.5 Sensitivity tests: exclusion ofhigh-performance and sporty vehicles 147 6.6 Linearity ofthe weight-safety relationships J£ 6.7 Sensitivity tests: concentrating the weight reductions on the heaviest vehicles .... 165 References Appendix A Valid VIN1-VIN3 combinations for 1981-93 vehicles 177 AppendixB Fundamental car groups, 1985-93 187 AppendixC Fundamental light truck groups, 1985-93 215 Appendix D Curb weight, track width and wheelbase of passenger cars 239 AppendixE Curb weight, track width and wheelbase of light trucks 251 Appendix F Summary and response to TRB's recommendations on the draft report 261 rv 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 ifthey 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 Admimstration (NHTSA) studied the safety effect ofthat weight and size reduction and concluded that it increased fatalities by nearly 2000 per year. Between 1985 and 1993, the number ofpassenger 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 ofanew 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 ofthe cars. That raises the question whether the growth mthe number and weight oflight trucks is having an adverse impact on the safety ofpassenger car occupants and other road users, possibly exceeding any safety benefits ofthe vehicle-weight increases forthe occupants ofthe trucks. The objective ofthis 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 ofthe "case" vehicle, but also the occupants ofother 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 ofthe fatalities in the United States: Principal rollovers (not resulting from acollision) Collisions with objects (e.g., impacts with trees) Collisions with pedestrians, bicycles, ormotorcycles Collisions with trucks over 10,000 pounds (Gross Vehicle Weight) Collisions with passenger cars Collisions with light trucks (pickups, SUVs, orvans) The results for light trucks are new, while the findings for passenger cars are acompletion and update ofNHTSA's 1991 study. The principal reason for analyzing cars again is that NHTSA's 1991 analysis did not address three types offetal 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 ofthis report supersede the 1991 findings for passenger cars. In view ofthe complexity and the high public interest in the issue of vehicle size and safety, adraft ofthis report was peer-reviewed by apanel

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