An Evaluation of Occupant Protection in Frontal Interior Impact for Unrestrained Front Seat Occupants of Cars And Light Trucks

Charles J. Kahane, Ph.D.

Federal Motor Vehicle Safety Standard 201, Occupant Protection in Interior Impact, regulates the performance of certain vehicle interior surfaces in crashes. During the 1960's and early 1970's, the manufacturers generally modified instrument panels of cars and light trucks, installing padding, reducing the rigidity of panel structures and extending the panel downward and toward the passenger. This evaluation analyzes the effectiveness and benefits of instrument panel improvements in frontal crashes of cars and light trucks. It also estimates the cumulative fatality reduction - for unrestrained front seat occupants in frontal crashes - for all safety standards and vehicle improvements of the 1964-84 era. The study is based on statistical analyses of FARS, NCSS and NASS accident data and MVMA2D computer simulations of occupants impacting the instrument panel in frontal crashes. It was found that:

• The instrument panel improvements of the 1965-75 era reduced fatality risk and serious injury risk by nearly 25 percent for unrestrained right front passengers of cars in frontal crashes, saving up to 700 lives per year.

• A preliminary analysis of light trucks indicates close to 25 percent reduction of fatality risk between model years 1968 and 1976, for unrestrained right front passengers in frontal crashes.

• All the passenger car standards and improvements of the 1964-84 period, other than safety belts, save about 2200 driver and front seat passenger fatalities per year in frontal crashes.

"Occupant Protection in Interior Impact" is the title of Federal Motor Vehicle Safety Standard 201. More generally. it is the synthesis of occupant compartment geometry, energy absorbing materials on the interior surfaces of the compartment and the Integrity and controlled crush of the entire vehicle structure. It is all the parts of a vehicle other than the restraint system - which, if well designed, combine to make the occupant compartment a potentially safe environment even in a severe crash.

The instrument panel is the single most important component for protecting the unrestrained right front passenger in a frontal crash. It is the large interior surface immediately in front of the passenger, whose knees are almost certain to contact the lower Instrument panel. The chest is likely to impact the mid panel and the head may rebound from the windshield and contact the top surface of the panel. Appropriate design and energy absorbing materials can lessen the injuries from these contacts. But the influence of the panel is not limited to these direct contacts by the passenger. A panel with appropriate geometry and force deflection characteristics can help keep the unrestrained passenger in an upright position during the crash and reduce the severity of the interactions with the windshield, the roof header and other components.

During the 1960's and 1970's, the manufacturers gradually modified the instrument panels of cars and light trucks in ways believed to reduce the injury risk for unrestrained right front passengers in frontal crashes. Instrument panel tops were padded in most cars by the mid 1960's. Subsequently, Standard 201 required the padding in all cars as of January 1, 1968 and in light trucks after September 1, 1981. The manufacturers gradually reduced the rigidity of mid and lower instrument panels (although Standard 201, as promulgated, does not set requirements in those areas). The panels were extended back further toward the passenger and the knee impact area enlarged. Softer, larger panels were believed to be helpful in reducing direct contact injuries and to decelerate the passenger more evenly over a longer time period ("ride down"), also keeping him in an upright position.

Executive Order 12291 (February 1981) requires agencies to evaluate their existing regulations. The objectives of an evaluation are to determine the actual benefits - lives saved, injuries prevented, damage avoided - and costs of safety equipment installed in production vehicles in connection with a standard. Standard 201 is the regulation on performance of the instrument panel during interior impacts. As explained above, though, many of the actual modifications of instrument panels were made well in advance of Standard 201 or were in areas of the panel not specifically covered by the standard. One objective of this report is to evaluate the cumulative reduction of fatalities and injuries of unrestrained right front passengers in frontal crashes as a result of all the instrument panel modifications that have been gradually made in cars and light trucks since the early 1960's. The study also takes a preliminary look at the correlation between injury severity, for various body regions, and certain parameters describing the geometry and force deflection characteristics of instrument panels.

By now, NHTSA has published evaluations of nearly all major safety devices regulated by its safety standards, especially those which protect unrestrained drivers and/or right front passengers of passenger cars in frontal impacts - e.g. energy absorbing steering assemblies and High Penetration Resistant windshields. Each of the previous evaluations gave an estimate of the number of lives saved by a particular safety device. That makes it appropriate to add a second objective to this "evaluation of occupant protection in interior impacts for front seat occupants in frontal crashes." The goal is to estimate the cumulative reduction in frontal fatality risk for unrestrained drivers and right front passengers of cars of the 1980's. relative to cars of the 1960's i.e., estimate the total of lives saved by all of the preceding safety devices combined plus the effects on crashworthiness of any other vehicle modifications that have not been evaluated or are not associated with a specific safety standard. For example, the change from rear wheel drive to front wheel drive in the 1980's is not connected to any particular safety standard but might nonetheless have safety implications if it affects vehicle crush characteristics. The analysis concludes the NHTSA evaluation of occupant protection in frontal crashes, addressing questions such as:

• What is the net contribution of the vehicle modifications made during the 1960-84 period? When did the reductions take place?

• Do the individual fatality reductions estimated for various safety standards in previous NHTSA evaluations add up to this evaluation's estimate of the overall reduction in fatalities from model year 1966 to 1969 (when most of those standards were implemented)?

• Did cars get any safer after 1970, thanks to improvements not necessarily related to NHTSA's standards?

The evaluation for passenger car occupants consists of three analyses. First, National Crash Severity Study (NCSS) data were statistically analyzed to determine the risk of serious injuries specifically due to contact with the instrument panel, by model year (1960-78), for unrestrained right front passengers in frontal crashes. The analysis controlled for confounding factors such as differences in the crash severities of older and newer cars.

But panel modifications, as stated above, can even affect some of the injuries not directly due to panel contact. The second analysis gauges the effect of panel design on the right front passenger's overall injury risk, based on simulation of 5th, 50th and 95th percentile passenger interactions with the vehicle interior in 25-30 mph frontal barrier crashes, using the MVMA2D computer model. Crashes are simulated with instrument panels having the geometry and force deflection characteristics of cars of a wide range of model years (1965-83) and body styles - but with all other vehicle factors, such as the crash pulse, the materials of the windshield, etc. held constant. The trend, by model year, of the injury criteria predicted by these simulations, is thus in a sense attributable to changes in the i nstrument panel , since everything else is held constant. The simulations also permit a preliminary correlational analysis of various types of injury with instrument panel characteristics. Of course, the computer simulations of this report, which for the most part were not validated by actual crash tests, need to be interpreted cautiously and in particular should not be used for predicting the injury risk in specific makes and models of cars - but a large sample of simulations gives a good idea of the historical trend of injury risk

The third analysis looks at the 16,000 fatal head on collisions of cars of two different model years on the Fatal Accident Reporting System (FARS) to see in each collision which driver is more likely to be killed - the one in the older car or in the newer car - taking into account such other factors such as the difference in vehicle weights, the drivers' ages, etc. The individual comparisons are combined into a model which predicts the unrestrained driver's fatality risk index as a function of model year, controlling for vehicle weight - and the decrease of this index from model year 1964 to 1984 estimates the cumulative reduction in frontal fatality risk, as a result of vehicle modifications (other than weight changes) during those years. The model is then extended to right front passengers. This approach using head on collisions eliminates most of the sources of bias that have often been present in earlier analyses to estimate fatality risk by model year: reporting biases, effects of factors other than vehicle modifications. When cars of two different model years collide head on, but with the same car weight, driver age, etc. and the fatalities occur consistently more often in the older car than in the newer one, the only conclusion is that the newer car is safer.

The study's most important results for unrestrained right front passengers of passenger cars are conveyed in Figures 1, 2 and 3. Figure 1 shows the relative risk of serious injury due to instrument panel contact, by model year (1960-78), based on the NCSS analysis. The average risk for 1971-78 cars is assigned a value of 100. Figure 2 shows the relative overall injury performance of instrument panels of different model years (1965-83) in the MVMA2D simulations. The measure of performance in Figure 2 cannot be translated into actual injury rates, but positive values mean higher injury risk and negative values, lower risk. Figure 3 shows the overall fatality risk index for unrestrained right front passengers in frontal crashes, by model year (1964-84). The average risk for 1973-84 cars is assigned the index value of 100. The three curves are derived from completely unrelated data sources and measure different types of risk, yet they all show nearly the same pattern: a large reduction (about 20 percent) of risk in cars of the later 1960's, followed by an additional smaller reduction (another 10 percent) in the early 1970's and a leveling off after that. These reductions coincide with the instrument panel modifications made by the manufacturers. It can be concluded that these modifications were effective in reducing injuries and fatalities - including, but not limited to the casualties specifically due to direct contact with the panel - and that a large proportion, if not most of the net reduction of overall fatality risk for right front passengers in frontal crashes is due to the panel modifications.

Figure 2: Effect Of Instrument Panel Designs On Overall Injury
Score In Computer Simulations Of Frontal Crashes

FIGURE 3: Overall Fatality Risk Index for Unrestrained Right Front Passengers in Frontal Crashes (Adjusted for Car Weight Changes: 1973-84 Average = 100)

Figure 4 shows the overall fatality risk index for unrestrained drivers in frontal crashes, by model year (1964-84). The average risk for 1973-84 cars is again assigned the index value of 100. Figure 4 shows a large reduction (about 12 percent) in model years 1967- 68, when energy absorbing steering assemblies were installed in passenger cars, with little net change from then on. The net difference between the 1964 and 1984 cars amounts to about 1300 driver fatalities per year - nearly the same as the reduction attributed by NHTSA's 1981 evaluation to the energy absorbing steering assembly. It seems that the energy absorbing steering assembly has been the vehicle modification of the 1964-84 period with the largest effect on unrestrained drivers' fatality risk in frontal crashes.

Figure 4: Overall Fatality Risk Index for Unrestrained Drivers in Frontal Crashes (Adjusted for Car Weight Changes: 1973-84 Average = 100)

The study of light trucks included statistical analyses of injury rates in National Accident Sampling System (NASS) and NCSS data and a calibration of fatality risk indices similar to those for passenger cars. Because sample sizes were smaller, the results were not nearly as conclusive as for passenger cars.

The least firm section of this report is its use of computer simulations, generally not verified by crash or sled tests, to compare the injury risk with instrument panels of different model years. While the simulations showed strong, intuitively reasonable correlations between certain types of panel design and high injury risk, it cannot be guaranteed that similar correlations would be found in real crashes. The FARS analysis for light trucks did not have precise curb weight data for the trucks. All of the NCSS, FARS and NASS analyses have relatively large sampling errors.

Although the evaluation concludes that instrument panel design has improved significantly since 1960, the panel still accounts for a large percentage of the serious injuries in frontal crashes. The major advances in biomechanics and simulation procedures during the past 10 years have encouraged NHTSA to undertake a research program on frontal protection for the right front passenger. An initial objective of that research is quantification of the injury consequences of changing various instrument panel design parameters - based in part on computer simulations which have been validated by crash or sled test data, a more accurate approach than the one used in this evaluation. The eventual goal is optimization of panel design.

Although the evaluation primarily investigates the safety of unrestrained occupants of cars of the 1960-84 era, it must not be forgotten that safety belts are the most important safety equipment introduced during that time. The effect of safety belts is not included in the fatality indices shown in Figures 3 and 4, but, regardless of the model year, belt users would have had a fatality risk far lower than unrestrained front seat occupants.

The principal findings and conclusions of the study are the following:

Principal Findings

Instrument panel as an injury source-in 1970-78 passenger cars

• 56 percent of unrestrained right front passengers in frontal crashes who have nonminor (AIS 2 or greater) injuries receive at least one of these injuries from contact with the instrument panel; 44 percent receive all of their nonminor injuries from panel contact.

• 45 percent of nonminor instrument panel contact injuries are torso injuries; 18 percent involve the head or neck; 37 percent, the legs or arms.

• 47 percent of unrestrained right front passenger fatalities in frontal crashes receive at least one life threatening (AIS 4 or greater) injury from contact with the instrument panel; 27 percent receive all of their life threatening injuries from panel contact.

Instrument panel design changes - based on measurements in actual cars

• The rigidity of mid and lower instrument panels decreased steadily from model year 1965 to 1977.

• The vertical-longitudinal periphery of the instrument panel - i.e., the distance from the bottom of the windshield to the back of the dashboard to the lowest point on the panel (a + b + c on the diagram at the beginning of the Summary) - increased steadily from model year 1965 to 1977.

• In 1965-66 cars, the mid instrument panel slopes down and away from the passenger. By 1979-, mid instrument panels were more nearly vertical or even sloped down and toward the passenger.

• The windshield is slightly more horizontal in cars of the late 1970's than in cars of the 1960's.

Instrument panel contact injury risk, by model year

• Let 100 be the average risk of nonminor (AIS 2+) injury due to instrument panel contact for unrestrained right front passengers in frontal crashes of 1971-78 model cars. The estimated risk index, by model year group is:

  Model Years	Relative Risk Factor	90% Confidence Bounds
  1960-66	140	109 to 171
  1967-70	107	81 to 134
  1971-74	90	76 to 104
  1975-78	110	85 to 136

• Injury risk is 23 percent lower in 1967-70 cars than in 1960-66 cars.

• Injury risk is 29 percent lower in 1971-78 cars than in 1960-66 cars.

• The reduction from 1960-66 to 1967-78 cars is statistically significant. The differences among the three later model year groups are nonsignificant.

Instrument panel design vs. injury risk, by model year (simulation results)

• Overall injury risk for unrestrained right front passengers in computer simulations of 25-30 mph frontal barrier crashes is significantly lower with 1969-71 and 1975-83 instrument panels than with 1965-66 panels. The 1975-83 panels may perform even slightly better than the 1969-71 panels.

• Head and neck injury risk, femur injury risk and chest g's are significantly lower with 1975-83 panels than with 1965-66 panels.

• Chest deflection, however, may be as severe or more severe with the 1975-83 panels than with the earlier panels.

Correlation of injury with instrument panel parameters (simulation results)

• A preliminary analysis of the computer simulations (which for the most part were not validated by actual crash tests) shows lower overall injury risk in the cars whose panels protruded toward the passenger and downwards (large vertical- longitudinal periphery) and whose lower instrument panels could be crushed for many inches before they became rigid.

• The least severe chest deflection was predicted in cars with soft mid instrument panels and hard lower instrument panels.

• The lowest chest g's were predicted in cars with soft lower instrument panels.

• This study uses a head injury score based to a large extent on HIC. More favorable head injury scores were found in simulations of cars with more nearly horizontal windshields and soft lower instrument panels (which help keep the passenger in an upright position during the crash).

• Femur loads were lowest in cars with soft lower instrument panels. The panels also had a large vertical-longitudinal periphery and the mid instrument panel did not slope downward towards the passenger's knees.

• Since the more recent cars had softer, longer panels than cars of the mid 1960's, it is appropriate that they had lower predictions for every type of injury except for inconclusive results on chest deflection.

Drivers' overall fatality risk index in frontal crashes, by model year

• Let 100 be the average fatality risk for unrestrained drivers in frontal crashes of 1973-84 model cars. The estimated risk index, by model year group is:

  Model Years	Relative Risk Index
  1964-66	117
  1968-70	103
  1971-74	100
  1975-78	102
  1979-81	100
  1982-84	101

The appropriate interpretation of the risk index is that if the fleet of 1964-66 cars had been replaced by a fleet of 1968-70 type cars with the same weights, driver ages, etc., there would have been only 103/117 as many driver fatalities in frontal crashes - i.e., a reduction of 12 percent.

• The fatality risk for unrestrained drivers is 12 percent lower in 1968-70 cars than in 1964-66 cars. It has remained almost constant since model year 1968.

• The 12 percent reduction in drivers' fatality risk in frontal crashes coincides with the installation of energy absorbing steering columns in 1967-68 cars - which an earlier NHTSA evaluation has credited with a 12 percent reduction of fatality risk in frontal crashes.

Passengers' overall fatality risk index in frontal crashes, by model Year

• Let 100 be the average fatality risk for unrestrained right front passengers in frontal crashes of 1973-84 model cars. The estimated risk index, by model year group is:

  Model Years	Relative Risk Index
  1964 - 66	136
  1968 - 70	109
  1971 - 74	106
  1975 - 78	98
  1979 - 81	97
  1982 - 84	103

• The fatality risk for unrestrained right front passengers is 20 percent lower in 1968-70 cars than in 1964-66 cars.

• The fatality risk for unrestrained right front passengers is 26 percent lower in 1973-84 cars than in 1964-66 cars.

• The big reduction of right front passengers' fatality risk in cars of the late 1960's, followed-by a further, more gradual reduction in the early to mid 1970's, coincides with the manufacturers' modifications of instrument panels.

• In cars of model years 1960-65, right front passengers' fatality risk in frontal crashes was 25 percent higher than for the drivers of the same cars. Since model year 1971, driver and right front passenger fatality risk have been about equal.

Lives saved per year by all frontal crashworthiness improvements in cars

• The fatality risk index for unrestrained drivers in frontal crashes dropped from 117 in 1964-66 cars to 100 in 1973-84 cars. That is equivalent to saving 1300 lives per year.

• The overall benefit for drivers can be apportioned as follows:

  	Lives Saved
  Energy absorbing steering assemblies	1100 - 1300
  All other vehicle modifications	0 - 200
  1300

• The fatality risk Index for unrestrained right front passengers in frontal crashes dropped from 136 in 1964-66 cars to 100 in 1973-84 cars. That is equivalent to saving 900 lives per year.

• The overall benefit for right front passengers can be apportioned as follows:

  	Lives Saved
  Instrument panel modifications	400 - 700
  Windshield glazing and mounting	100 - 300
  All other vehicle modifications	0 - 400
  900

• All passenger car modifications of the 1964-84 period, other than restraint systems, save a total of 2,200 front seat occupant fatalities per year in frontal crashes.

Light trucks: injury risk by model year

• The NASS and NCSS files do not contain enough cases for finding meaningful trends in the nonminor injury rate in frontal crashes of light trucks, vans and multipurpose passenger vehicles. The observed injury rates are:

  Model Years	Right Front Passengers	Drivers
  	NASS AIS 2+ Injury Rate ( %)
  1966 - 70	12.9	9.7
  1971 - 74	7.4	7.5
  1975 - 78	9.1	8.2
  1979 - 81	12.3	10.7
  1982 - 85	10.9	12.3
  	NCSS Hospitalization Rate (%)
  1961 - 70	11.4	6.0
  1971 - 74	7.6	10.8
  1975 - 78	14.1	10.1

Light trucks: drivers' fatality risk in, frontals, by model year

• Let 100 be the average fatality risk for unrestrained drivers in frontal crashes of 1973-84 model light trucks, vans and multipurpose passenger vehicles. The estimated risk index, by model year group is:

  Model Years	Relative Risk Index
  1964 - 68	135
  1969 - 72	114
  1973 - 76	106
  1977 - 81	97
  1982 - 84	97

• The fatality risk for unrestrained right front passengers is 15 percent lower in 1977-81 trucks than in 1969-72 trucks and 28 percent lower than in 1964-68 trucks. It has remained almost constant since model year 1977.

• Throughout model years 1960-84, right front passengers, fatality risk in frontal crashes was consistently close to 10 percent lower than for the drivers of the same trucks.

  Conclusions

• Instrument panel modifications, implemented by manufacturers on a voluntary basis during the later 1960's and early to mid 1970's, have significantly reduced the fatalities and serious injuries of right front passengers in frontal crashes.

• The safety literature of 1968-70 claims that instrument panels were becoming softer and extending further toward the passenger and the floor. Tests and measurements of instrument panels in production vehicles of the 1965-80 era show the claims are correct.

• The safety literature of 1968-70 claims that softer and more extensive instrument panels reduce injury risk by cushioning direct impacts, providing better ride down and keeping the passenger in an upright position during the crash. The computer simulations of crashes with production instrument panels support all of these claims.

• Cars of the 1970's are significantly safer than cars of the same weight from the mid 1960's for unrestrained front seat occupants in frontal crashes. But cars of the mid 1980's are about as safe in frontal crashes as cars of the same weight from the mid 1970's.

• Energy absorbing steering assemblies meeting Standards 203 and 204 are responsible for most of the improvement in frontal crashworthiness for unrestrained drivers in cars of the 1964-84 era.

• The manufacturers' voluntary improvements to instrument panels are responsible for most of the improvement in frontal crashworthiness for unrestrained right front passengers in cars of the 1964-84 era. Windshield modifications meeting Standards 205 and 212 account for a smaller share of the fatality reduction.

• No firm conclusions can be drawn on the frontal crashworthiness of light trucks. The preliminary analysis of fatal accident data showed promising reductions of risk during the early to mid 1970's, a time of major safety improvements to light trucks. But the small samples of nonfatal accident data do not show similar trends in the injury rates.