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

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.

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

• Back seat outboard belt use is substantially lower than front seat outboard belt use. Front seat belt use in 1997 was almost 70 percent whereas back seat belt use was 40 percent.
• Back seat outboard belt use is higher in cars equipped with lap/shoulder belts than cars with lap belts. Belt use is 7-10 percentage points higher in cars with lap/shoulder belts than cars with lap belts after controlling for vehicle age and calendar year.

FATALITY REDUCTION - BACK SEAT OUTBOARD BELTS - PASSENGER CARS

• In all crashes, back seat lap belts are 32 percent effective in reducing fatalities when compared to unrestrained back seat occupants. The effectiveness estimate is statistically significant with confidence bounds: 23 to 40 percent.
• In all crashes, back seat lap/shoulder belts are 44 percent effective in reducing fatalities when compared to unrestrained back seat occupants. The effectiveness estimate is statistically significant with confidence bounds: 38 to 50 percent.
• In all crashes, back seat lap/shoulder belts are 15 percent effective in reducing fatalities when compared to back seat lap belts. The effectiveness estimate is statistically significant with confidence bounds: 5 to 25 percent.
• At current belt use rates in the back seat, a passenger car fleet equipped entirely with lap/shoulder belts will have approximately 124 fewer fatalities per year than a fleet equipped only with lap belts (confidence bounds: 63 to 180).
• If belt use in the back seat increased to 100 percent, lap/shoulder belts could save an additional 515 lives.

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

• In frontal crashes, lap belted back seat outboard occupants did not have a statistically significant benefit relative to unrestrained occupants. The effectiveness estimate is 1 percent.
• Lap/shoulder belts are 29 percent effective in reducing fatalities when compared to unrestrained occupants in frontal crashes. This estimate is statistically significant with confidence bounds: 15 to 42 percent.
• In frontal crashes, the lap/shoulder belt is a great improvement over the lap belt. Lap/shoulder belts are 25 percent effective in reducing fatalities when compared to lap belted occupants in frontal crashes. This estimate is statistically significant with confidence bounds: 12 to 36 percent.

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

• In all types of non-frontal crashes, including side impacts, rear impacts, rollovers, and others, lap/shoulder belts are perhaps slightly more effective in reducing fatalities than lap belts, but the increment is not statistically significant:

• lap belted vs. unrestrained back seat outboard occupants: 48 percent.
• lap/shoulder belted vs. unrestrained back seat outboard occupants: 53 percent.
• lap/shoulder belted vs. lap belted back seat outboard occupants: 6 percent.
• In rollovers, lap and lap/shoulder belts when used are both highly effective in preventing a fatality: 76 and 77 percent, respectively. In rollovers, the lap/shoulder belt does not provide any significant additional protection over a lap belt.
• In side impact crashes, the lap/shoulder belt when used provides about the same protection against a fatality as a lap belt.

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

• Children ages 5-14 appear to derive the greatest incremental benefit from using back seat lap/shoulder belts rather than just a lap belt. The incremental effectiveness of lap/shoulder belts is 26 percent for children.
• It is unclear if males age 15-54 benefit more from being lap/shoulder belted than just lap belted.
• Females age 15-54 appear to benefit slightly from lap/shoulder belts over lap belts.
• Seniors ages 55 and over benefit from using back seat lap/shoulder belts over lap belts, but neither lap nor lap/shoulder belts when used are as beneficial for seniors as they are for the other age/gender groups.
• In frontal crashes, both lap and lap/shoulder belts have consistently lower effectiveness estimates as occupants get older when compared to unbelted occupants. However, the relative incremental benefit of lap/shoulder belts over lap belts remains reasonably consistent, ranging from 25 to 41 percent.
• Gender also appears to affect the effectiveness estimates in frontal crashes. Females age 15-54 have lower effectiveness estimates than males age 15-54.

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

• Back seat outboard belts are highly effective is reducing fatalities when compared to unrestrained occupants in passenger vans and SUVs. Lap belts are 63 percent effective and lap/shoulder belts are 73 percent effective. Belts are so effective in these vehicles because they eliminate the risk of ejection, a big problem for unrestrained occupants in these vehicles.
• The effectiveness of the lap/shoulder belt relative to the lap belt in passenger vans and SUVs is 20 percent. This estimate is not statistically significant, but it is close to the corresponding estimate for passenger cars.
• In frontal crashes, both the lap and lap/shoulder belts are highly effective in passenger vans and SUVs. Ejection is also a big problem in frontal crashes for these vehicles, so even the lap belt is effective in frontal crashes. But the lap/shoulder belt is not significantly safer than the lap belt in frontal crashes for these vehicles.

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:

• Lap belt use in frontal crashes increases the risk of abdominal injuries even while reducing the risk of head injuries relative to unrestrained occupants.
• Lap/shoulder belts reduce abdominal injuries by 52 percent relative to lap belts only.
• Lap/shoulder belts reduce head injuries by 47 percent relative to lap belts only.
• The shoulder belt does not appear to increase the risk of chest injuries. The fatality rate due to chest injuries for lap/shoulder belted occupants is only slightly higher than the rate for lap belts, which is slightly higher than unrestrained occupants.
• Belted children, ages 5-14, do not have an increased risk of abdominal injuries. Lap belted and lap/shoulder belted children have abdominal injury rates slightly higher than unrestrained children in frontal crashes, but at or below the injury rate of all lap belted and lap/shoulder belted occupants in frontal crashes.
• Lap belted seniors (aged 55 and older), males (ages 15-54), and females (ages 15-54) have an increased risk of abdominal injuries in frontal crashes. However, the risk of abdominal injuries for lap belted females is lower than the risk for males and seniors.