NHTSA Report Number DOT HS 808 945June 1999

Effectiveness of Lap/Shoulder Belts in the Back Outboard Seating Positions

Christina Morgan

Abstract

Most of the analyses are based on Fatality Analysis Reporting System (FARS) data from 1988 through the first six months of 1997. The primary analysis compares the fatality risk for back seat outboard belted occupants (lap or lap/shoulder belted) to the corresponding risk for unbelted occupants, as well as the fatality risk for lap/shoulder belted occupants to the risk for lap belted occupants. Fatality risk is the ratio of fatalities in the back seat to fatalities in the front seat (a control group). This procedure of comparing a subject group to a control group is called "double pair comparison." The principal conclusions are: back seat lap belts are 32 percent effective in reducing fatalities and lap/shoulder belts are 44 percent effective in reducing fatalities when compared to unrestrained back seat occupants in passenger cars. In passenger vans and Sport Utility Vehicles, lap belts are 63 percent effective and lap/shoulder belts are 73 percent effective. The change from lap to lap/shoulder belts has significantly enhanced occupant protection, especially in frontal crashes. In all crashes, lap/shoulder belts are 15 percent more effective than lap belts alone. In frontal crashes, lap/shoulder belts are 25 percent more effective than lap belts alone. Back seat lap belts reduce the risk of head injuries while increasing the risk of abdominal injuries in potentially fatal frontal crashes. Lap/shoulder belts reduce the risk of both head and abdominal injuries in potentially fatal frontal crashes relative to lap belts only: head injuries by 47 percent and abdominal injuries by 52 percent.

Executive Summary

The objective of this report is to evaluate back seat outboard lap/shoulder belts. This report evaluates the effectiveness of lap/shoulder belts for back seat outboard occupants and whether they are more effective than lap belts for these occupants. A controversial study by the National Transportation Safety Board (NTSB) claimed that lap belts are ineffective and possibly dangerous and that only lap/shoulder belts could protect the occupants. Other studies, while not disputing that lap/shoulder belts would be superior, found benefits for lap belts. A second objective of this evaluation is to determine whether lap belts are effective, whether lap belts are harmful to back seat belt users, and whether lap/shoulder belts correct the problems found with lap belts.

Agencies are required to evaluate their existing programs and regulations by the Government Performance and Results Act of 1993 and Executive Order 12866 (October 1993). This report is an evaluation of the Federal Motor Vehicle Safety Standard (FMVSS) 208 (occupant crash protection), specifically the back seat lap/shoulder belt requirement. Back seat outboard lap/shoulder belts were first required in passenger cars after December 11, 1989 and in convertible passenger cars, light trucks, vans, and sport utility vehicles (SUVs) after September 1, 1991. Before this, passenger vehicles were required to have at least lap belts at all forward-facing rear outboard seating positions, although lap/shoulder belts would also have met this requirement. By model year 1990, all passenger cars, except convertibles, had lap/shoulder belts at the rear outboard seating positions. Similarly, many passenger vans and SUVs were equipped with lap/shoulder belts in the back outboard seating positions before the FMVSS 208 requirement.

Most of the analyses are based on Fatality Analysis Reporting System (FARS) data from 1988 through the first six months of 1997. FARS is a census of fatal crashes in the United States. Several different analytical methods are used to assess the effectiveness of back seat outboard lap and lap/shoulder belts. Our primary analysis compares the fatality risk for belted occupants (lap and lap/shoulder belted) to the corresponding fatality risk for unbelted occupants, as well as the fatality risk for lap/shoulder belted occupants to the fatality risk for lap belted occupants ("when-used" analysis). Fatality risk is the ratio of fatalities in the back seat to fatalities in the front seat (a control group). This procedure of comparing a subject group to a control group is called "double pair comparison."

The principal conclusions of the study are that lap belts and lap/shoulder belts are effective for back seat outboard occupants. The change from lap to lap/shoulders has significantly enhanced occupant protection, especially in frontal crashes. Back seat lap belts reduce the risk of head injuries while increasing the risk of abdominal injuries in frontal crashes. Lap/shoulder belts reduce the risk of both head and abdominal injuries in frontal crashes relative to lap belts only. Lap/shoulder belts reduce abdominal injuries by 52 percent relative to lap belts only. A passenger car fleet entirely equipped with lap/shoulder belts will have approximately 124 fewer fatalities per year than a fleet equipped only with lap belts. The principal findings and conclusions are the following:

BELT USE BY BACK SEAT OUTBOARD OCCUPANTS

FATALITY REDUCTION - BACK SEAT OUTBOARD BELTS - PASSENGER CARS

FATALITY REDUCTION IN FRONTAL CRASHES - BACK SEAT OUTBOARD BELTS - PASSENGER CARS

FATALITY REDUCTION IN NON-FRONTAL CRASHES - BACK SEAT OUTBOARD BELTS - PASSENGER CARS

FATALITY REDUCTION BY AGE AND GENDER GROUPS - BACK SEAT OUTBOARD BELTS - PASSENGER CARS

FATALITY REDUCTION - BACK SEAT OUTBOARD BELTS - PASSENGER VANS AND SUVS

FATAL INJURY RATE BY BODY REGION IN FRONTAL CRASHES

By combining the FARS and the Multiple Cause of Death (MCOD) files, we found the causes of death listed on the death certificate of fatal crash victims. We analyzed this file and found the following regarding causes of death by body region injured:

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