Report No. DOT HS 810 685

Charles J. Kahane, Ph.D.

August 2006

ABSTRACT

The first generation of frontal air bags saved the lives of thousands of drivers and adult or teenage right-front passengers.  But they harmed occupants positioned close to the air bag at the time of deployment, especially infants and children.  In 1998-1999, air bags were redesigned by depowering – by removing some of the gas-generating propellant or stored gas from their inflators – and/or by reducing the volume or rearward extent of air bags, positioning them further from occupants, tethering and hybrid inflators.  NHTSA facilitated the redesign by permitting a sled test in lieu of a barrier impact to certify that air bags would protect an unrestrained occupant (“sled certification”).  Statistical analyses of crash data through 2004 from NHTSA’s Fatality Analysis Reporting System (FARS) and the Special Crash Investigations (SCI) compare fatality risk with sled-certified and first-generation air bags.

• The overall fatality risk in frontal crashes of 0-12 year-old child passengers in the front seat is a statistically significant 45 percent lower with sled-certified air bags than with first-generation air bags; fatalities caused by air bags in low-speed crashes were reduced by 83 percent.
• The overall fatality risk of drivers and of right-front passengers age 13 and older in frontal crashes is not significantly different with sled-certified air bags than with first-generation air bags; sled-certified air bags preserved the life-saving benefits of first-generation air bags.  

EXECUTIVE SUMMARY

Frontal air bags saved the lives of an estimated 16,905 drivers and adult or teenage right-front passengers from 1987 through the end of 2004.  But they can harm occupants positioned close to the air bag at the time of deployment.  The National Highway Traffic Safety Administration (NHTSA) had long warned consumers not to place an infant’s rear-facing safety seat in the front seat of a vehicle equipped with passenger air bags, because the infant would be close to the air bag at all times. 

By late 1995, it was evident that not only infants, but also children and even some adults were injured by air bags; worse, statistical analyses showed a significant increase in overall fatality risk with air bags for children age 0-12 years (but a significant net benefit for adults and teenagers).  Children slid forward toward the air bag during pre-crash braking, especially if they were unrestrained or on the lap of another passenger.  Short drivers often sat close to the air bag located in the steering wheel.  Any occupant could approach an air bag by leaning forward, for instance, to adjust the radio or air-conditioning.  By late 1995, NHTSA had identified 30 fatalities due to contact with air bags in otherwise survivable crashes (that number had grown to 264 by the end of 2005).

Therefore, beginning in October 1995, NHTSA initiated immediate, interim and long-term actions to reduce and eventually eliminate the adverse effect of air bags for infants, children and other high-risk occupants while retaining, to the largest extent possible, the great life-saving benefits of air bags for most people:

• An immediate campaign to inform motorists of dangers from air bags, urging that children travel in the back seat.  On-off switches for passenger air bags were factory-installed in pickup trucks without back seats.  Subsequently, NHTSA also advised drivers to sit at least 10 inches away from the air bag.  Drivers at high risk or people who must transport high-risk individuals in the front passenger seat could obtain aftermarket on-off switches at their own expense.
• An interim effort to furnish, as soon as possible, air bags that deploy less forcefully.  On March 19, 1997, NHTSA modified the frontal impact test for the unrestrained dummy in Federal Motor Vehicle Safety Standard (FMVSS) No. 208 – “Occupant Crash Protection.”  That facilitated an industry-wide changeover, using already available technologies, to “redesigned” air bags in the very next model years (1998-1999).

·        A long-term effort to develop “advanced” air bags that do not deploy at all for children (“suppression”), deploy only at a low level of force (“low-risk deployment”), or track an occupant’s motion and suppress the air bag if they are too close (“dynamic automatic suppression”).  Advanced air bags were phased into the fleet starting September 1, 2003 and were required in all new vehicles by September 1, 2006. 

The interim effort – the redesign of frontal air bags in 1998-1999 – is the focus of this report.  In the 1980s, the vast majority of fatalities were unrestrained, and the first generation of air bags was designed to protect unrestrained occupants as well as restrained occupants in frontal crashes.  Air bags had to pass two 30 mph crash tests, where the vehicle itself impacts a rigid barrier, with unrestrained dummies in the driver’s and right-front passenger’s seats on one of the tests and restrained dummies on the other.  Air bags had to fill up quickly with enough gas to cushion a large, unrestrained occupant, resulting in some designs having high initial pressure and velocity – and high injury risk for any person who might have strayed into the first two or three inches of the deployment path.  By 1995-1996, serious injuries to children needed immediate remedy, whereas the great increase in belt use, with buckle-up laws in almost every State, had shifted priority away from the unrestrained occupant. 

An imminently available technology to reduce the initial pressure and velocity of deployments was to “depower” air bags by removing some of the gas-generating propellant or stored gas from their inflators.  Other innovations already in progress included reducing the volume or rearward extent of air bags, positioning them further from occupants, revised folding techniques, tethering and shifting from pyrotechnic inflators to hybrids including stored gas.  The “redesign” of air bags in 1998-1999 consisted of depowering and/or some of the other innovations.

The manufacturers, however, doubted that depowered air bags would readily meet the existing FMVSS No. 208 crash test with unbelted dummies at 30 mph into a rigid barrier.  The American Automobile Manufacturers Association proposed that the 30 mph unbelted crash test be optionally replaced with a sled test, also at 30 mph, with a standardized crash pulse lasting 125 milliseconds, a substantially more gradual deceleration than the barrier test on a typical vehicle.  NHTSA and the safety community agreed that the proposal would facilitate the immediate redesign of air bags.  NHTSA began to allow the sled test as an option on March 19, 1997.  Make-models became “sled-certified” when the manufacturer chose the sled test option (whether or not the air bag was actually redesigned).  Make-models comprising approximately 70 percent of sales were sled-certified for the entire 1998 model year.  By the beginning of the 1999 model year, about 99 percent were sled-certified.  

In other words, not every sled-certified air bag is necessarily depowered or redesigned, and vice-versa.  But the overlap is great.  For the vehicles where NHTSA has full information about air bag performance, 84 percent of the driver air bags were depowered upon sled certification and 2 percent, although not depowered, had some other feature substantially redesigned.  On the passenger side, 70 percent were depowered upon sled certification and 11 percent were substantially redesigned in some other way, most often by shifting from pyrotechnic to hybrid inflators. 

Because NHTSA knows when almost every make-model was sled-certified, but has only partial information on depowering and other redesign, the analyses in this report primarily compare the fatality risk in sled-certified vehicles to the risk in the model years just before sled certification.

Before it allowed sled certification, NHTSA tested the performance of depowered air bags and analyzed data to quantify the likely impact on safety:

• Out-of-position occupants: Based on deployment tests with dummies located close to the air bag, NHTSA was confident that depowering would prevent a lot of the fatalities with first-generation air bags.  The agency anticipated a 44 percent reduction of fatalities caused by air bags to children age 1-12 years, and a large but unquantifiable reduction for out-of-position drivers and adult passengers.  The agency did not estimate any specific benefit from depowering for infants in rear-facing seats.
• Correctly positioned drivers and adult passengers:

Unrestrained: Substantial gas pressure is needed to cushion an unbelted occupant’s thorax.  Depowering, intuitively, could allow some unrestrained adults to “punch through” the air bag and be more severely injured by the structures beneath it, such as the steering assembly.  Tests, however, did not always show increased risk.  The agency anticipated a fatality increase ranging from near zero to possibly as high as 16 percent.

• Belted: “Punch-through” is less likely because safety belts absorb much of the occupant’s kinetic energy, and a less aggressive air bag might help in certain crashes.  Tests did not show a clear directional effect.  The agency anticipated little net change in fatality risk, ranging from a 2 percent reduction to a 3 percent increase.

This report analyzes crash data up to calendar year 2004 to find what actually happened to fatality risk after air bags were sled-certified.  NHTSA’s Special Crash Investigations (SCI) identify the fatalities to infants, children and out-of-position adults due to contact with air bags in otherwise survivable, low-to-moderate speed crashes (defined as less than 25 mph Delta V): “SCI fatalities.”  The Fatality Analysis Reporting System (FARS), a census of fatal crashes, provided data to compare overall fatality risk in frontal crashes before and after sled certification. 

The results are quite positive.  Sled-certified air bags greatly reduced SCI fatalities to infants, children and adults, even beyond NHTSA’s expectations.  They did not completely eliminate that risk.   Children up to and including age 12 years should continue riding in the back seat to avoid harm from air bags.  In pickup trucks without a back seat but with an on-off switch available for the passenger air bag, that switch should be turned off when a child passenger up to and including age 12 years is riding in the front seat. 

Sled-certified air bags entirely preserved the great life-saving benefits of first-generation air bags for belted drivers and for passengers age 13 years and up.  Even for unbelted drivers and passengers, overall fatality risk did not change significantly relative to first-generation air bags. 

In short, the combination of redesigned air bags and campaigns urging that children travel in the back seat reduced the SCI fatality rate for infants and children by 93 percent without any significant overall side effect for adults.  It has been a highly successful interim measure until advanced air bags could be developed and placed in service.  

The findings of the statistical analyses are the following:

THE SAFETY PROBLEM – IN PERSPECTIVE

·        Before sled certification, a large percentage of the passenger fatalities in frontal crashes among children up to age 10 years were caused by air bags in low-to-moderate speed crashes and would have been survivable without the air bag (“SCI fatalities”).  Here is a table of the occupant fatalities at seat positions equipped with first-generation air bags (not sled-certified) in calendar years 1990-2003:

                                                                               SCI                  All Frontal                   %

                                                                             Fatals                    Fatals                      SCI

Child passengers (age 0-12)                                  146                        351                      41.60

        Less than 1 year                                                 27                          42                      64.29

           1 –   5 years                                                     80                        158                      50.63

          6 – 10 years                                                     37                        117                      31.62

        11 – 12 years                                                       2                          34                        5.88

Adult passengers (age 13+)                                       9                     3,899                          .23

Drivers                                                                     78                   29,361                          .27

        Unbelted females < 5’3”, age 70+                         7                        180                        3.88

• Less than 1 percent of fatalities to drivers and to passengers age 13 years and up were SCI fatalities.

·        Even for the most vulnerable group of drivers – unrestrained females up to 5’3” tall and at least 70 years old – only 4 percent of frontal fatals were SCI cases, about the same as for pre-teen passengers age 11-12 years.

MANUFACTURER RESPONSE TO SLED CERTIFICATION

• The extent of depowering may be described by the percent reduction in two parameters:

The peak pressure of the deploying air bag

The average rise rate of the pressure as it approaches its peak.

• The peak pressure of air bags decreased by an average of 13 percent on the driver’s side upon sled certification, and likewise by 13 percent on the passenger side.

·        The rise rate decreased by an average of 24 percent on the driver’s side upon sled certification, and by 18 percent on the passenger side. 

EFFECT OF SLED-CERTIFICATION ON SCI FATALITIES OF CHILDREN

·        Here is a table of child passenger fatalities age 0-12 years caused by air bags in low-to-moderate speed crashes that would have been survivable without the air bag (“SCI fatalities”) – per billion vehicle years, during calendar years 1998-2003.  The year-to-year changes in children’s front-seat occupancy are not a factor because the table compares SCI fatality rates during the same calendar years:

Calendar                                                              SCI                   Vehicle              SCI Fatalities

    Years                       Vehicles                         Fatalities                 Years             per Billion Years

1998-2003           Not sled-certified                        75                213,481,330                353.0

1998-2003           Sled-certified                              15                243,523,549                  61.6

• For all children age 0-12 years in the front seat, the fatality rate in sled-certified vehicles was 83 percent lower than in the vehicles with first-generation air bags, during the same time period (calendar years 1998-2003).
• During 1998-2003 there were 11 SCI fatalities to infants in rear-facing safety seats in vehicles with first-generation air bags, none with sled-certified air bags.
• For 0-5 year-old children not in rear-facing seats, the SCI fatality rate in calendar years 1998-2003 was 78 percent lower with sled-certified air bags than with first-generation air bags; for 6-10 year-old children, 82 percent lower.
• Actual SCI fatality reductions for children exceed the 44 percent anticipated by NHTSA.
• During 1998-2003 there were 11 SCI fatalities to children age 0-12 years in vans with first-generation air bags, none with sled-certified air bags.

·        For 0-12 year-old children, the SCI fatality rate in calendar years 1998-2003 was reduced with sled-certified air bags by 78 percent in passenger cars and by 93 percent in SUVs.

EFFECT OF SLED-CERTIFICATION ON SCI FATALITIES OF DRIVERS

·        Drivers’ SCI fatality rate in calendar years 1998-2003 was 70 percent lower with sled-certified air bags than with first-generation air bags.

EFFECT OF SLED-CERTIFICATION ON SCI FATALITIES OF ADULT AND TEENAGE PASSENGERS

·        The SCI fatality rate of right-front passengers age 13 years and up in calendar years 1998-2003 was 42 percent lower with sled-certified air bags than with first-generation air bags.

COMBINED EFFECT OF SLED-CERTIFICATION AND MOVING TO THE BACK SEAT ON CHILDREN’S SCI FATALITIES

·        For all children age 0-12 years, the fatality rate in sled-certified vehicles during calendar years 1998-2003 was 93 percent lower than it had been in vehicles with first-generation air bags, during calendar years 1990-1997. 

Calendar                                                              SCI                   Vehicle              SCI Fatalities

    Years                       Vehicles                         Fatalities                 Years             per Billion Years

1990-1997           Not sled-certified                        65                  73,305,874                886.7

1998-2003           Sled-certified                              15                243,523,549                  61.6

·        That reduction is the combined effect of safer air bags and behavioral changes over time: moving children to the back seat, and if children stayed in the front seat, at least moving the seat all the way back and increasing use of restraints.

EFFECT OF SLED-CERTIFICATION ON CHILDREN’S OVERALL FATALITY RISK IN FRONTAL IMPACTS

• The fatality risk in frontal crashes of 0-12 year-old child passengers in the front seat is a statistically significant 45 percent lower with sled-certified air bags than with first-generation air bags (90 percent confidence bounds: 30 to 56 percent).
• Fatality reductions were substantial for every type of vehicle:

                                                                                                 Frontal Fatality Reduction (%)

                                              Vehicle Type                                 with Sled-Certified Air Bags

                                              Passenger car                                                   30

                                               Pickup truck                                                     55

                                              SUV                                                                48

                                              Van                                                                  89

·        Fatality reductions were larger for the younger children:

                                                    Age of the                             Frontal Fatality Reduction (%)

                                                Child Passenger                           with Sled-Certified Air Bags

                                              Less than 1 year                                               59

                                                 1 –   5 years                                                   58

                                                6 – 10 years                                                   31

                                              11 – 12 years                                                     8

·        The results are consistent with the great reduction of SCI fatalities with sled-certified air bags.  For children up to age 10 years, approximately half the fatalities in frontal crashes with original air bags were SCI fatalities.

EFFECT OF SLED-CERTIFICATION ON DRIVERS’ OVERALL FATALITY RISK IN FRONTAL IMPACTS

• The overall fatality risk of drivers in frontal crashes is the same with sled-certified air bags as with first-generation air bags (95 percent confidence bounds range from a 5 percent reduction to a 4 percent increase in fatality risk with sled-certified air bags).
• The fatality risk of belted drivers in frontal crashes is 5 percent lower with sled-certified air bags than with first-generation air bags.  The reduction is not statistically significant.
• The fatality risk of unrestrained drivers in frontal crashes is 5 percent higher with sled-certified air bags than with first-generation air bags.  The increase is not statistically significant.
• Sled-certified air bags preserved the life-saving benefits of first-generation air bags for drivers.

EFFECT OF SLED-CERTIFICATION ON ADULT AND TEENAGE PASSENGERS’ OVERALL FATALITY RISK IN FRONTAL IMPACTS

• The overall fatality risk of right-front passengers age 13 years and older in frontal crashes is 5 percent lower with sled-certified air bags than with first-generation air bags.  The reduction is not statistically significant (95 percent confidence bounds range from a 13 percent reduction to a 5 percent increase in fatality risk with sled-certified air bags).
• The fatality risk of belted right-front passengers in frontal crashes is 5 percent lower with sled-certified air bags than with first-generation air bags.  The reduction is not statistically significant.
• The fatality risk of unrestrained right-front passengers in frontal crashes is 7 percent lower with sled-certified air bags than with first-generation air bags.  The reduction is not statistically significant.
• Sled-certified air bags preserved the life-saving benefits of first-generation air bags for front-seat passengers age 13 years and up.