|NHTSA Report Number DOT HS 805 705||January 1981|
Charles J. Kahane, Ph.D.
Energy absorbing steering columns were installed in passenger cars in response to Federal Motor Vehicle Safety Standard 203. The columns are designed to compress at a controlled rate, cushioning the impact of the driver's chest in frontal crashes. Standard 204 specifies requirements limiting the rearward displacement of the steering wheel toward the driver. The objectives of this Agency evaluation are to determine how many driver fatalities and injuries are prevented by Standards 203 and 204, to measure the actual cost of the standards, to assess cost effectiveness and to describe the actual crash performance of equipment installed in response to the standards. The evaluation is based on statistical analyses of Fatal Accident Reporting System and National Crash Severity Study data, cost analyses of actual steering assemblies and a review of laboratory and crash tests and multidisciplinary accident investigations. It was found that
The steering assembly is the most common source of serious injury for drivers involved in frontal crashes. In passenger cars built before the 1967 model year, the steering column was a rigid pole ending in a narrow hub. In frontal crashes, the driver would hit the rigid column, his load concentrated on the narrow hub. Even worse, in some crashes the steering column was propelled rearwards, toward the driver, at a high rate of speed. Steering wheels and spokes were weak and brittle and contained hazardous metal attachments.
During the 1960's, the motor vehicle manufacturers, in cooperation with the. safety research community, developed energy absorbing columns that collapsed at a controlled rate when the driver hit them. Methods were discovered to prevent the rearward displacement of the column in crashes and safer steering wheels were designed. The General Services Administration established criteria for testing the performance of the improved steering assemblies under controlled conditions. These performance criteria became Standard 515/4a for Government vehicles. In 1967, the National Highway Traffic Safety Administration extended the requirements to all passenger cars sold in the United States, effective January 1, 1968. The requirements were promulgated as Federal Motor Vehicle Safety Standards 203 and 204. All passenger cars since model year 1968, as well as many 1967 models, appear to have met the Standards. In addition, the manufacturers have voluntarily made some improvements in the steering wheels that were not strictly required for compliance with the Standards.
Executive Order 12044 (March 1978) and Department of Transportation Order 2100.5 (May 1980) called for a review and evaluation of existing major regulations. This study is an evaluation of the vehicle modifications made in response to Standards 203 and 204, based on the actual operating experience of passenger cars. The evaluation objectives are
(1) Calculating the overall benefits of the vehicle modifications - life savings and injury severity reduction - treating Standard 203, Standard 204.and the voluntary steering wheel improvements as a single unit.
(2) Measuring the actual cost of the modifications.
(3) Assessing cost-effectiveness.
(4) Comparing the compliance test requirements to the performance of post-Standard vehicles in highway accidents.
(5) Explaining why the Standards have been effective; assessing the benefit for each specific vehicle modification and the mechanism whereby it produces benefits.
(6) Identifying the principal shortcomings of the current Standards vehicle improvements whose benefits did not meet expectations.
(7) Identifying areas in which Standards 203 and 204 could potentially be improved.
The fatality reduction due to Standards 203 and 204 was estimated by analyzing 5 years of Fatal Accident Reporting System (FARS) data. Statistical analyses of National Crash Severity Study (NCSS) data - 11,840 accident cases were on file as of November 1979 - were performed to determine the number of serious injuries prevented. The Multidisciplinary Accident Investigation (MDAI) file provided information on steering column compression. The cost of Standards 203 and 204 was calculated by analyzing the individual components of a representative sample of steering assemblies.
The results from the FARS, NCSS and MDAI analyses were compared to previously published statistical studies of Standards 203 and 204. Laboratory and crash test results were reviewed, as were clinical analyses of selected accident cases. The research, rulemaking and enforcement activities related to the two Standards were discussed with Agency engineers. The conclusions of this evaluation are based on all of the information sources - statistical, clinical and engineering.
The most important and definitive conclusions of this evaluation are that Standards 203 and 204 have reduced the number of driver fatalities and serious injuries in frontal crashes. Standard 204 has decreased rearward displacement of the steering column. These conclusions are based on statistically significant and consistent findings from a wide variety of data files. The statistical findings, moreover, were uniformly consistent with engineering intuition and clinical analyses.
The findings on some of the detailed analyses, such as the effectiveness of specific types of energy-absorbing devices, were not statistically significant because they involved splitting the data into subsamples. Conclusions based on those findings are less than definitive.
The conclusions on why the Standards have been effective, how much each hardware improvement has contributed to benefits and what could be done to enhance effectiveness must be considered speculative. These conclusions are intuitive judgments based on a thorough review of engineering analyses, selected accident cases, test results and statistical tabulations.
The evaluation suffers from the inherent shortcoming of a "before-after" design. The pre-Standard cars - model year 1967 and earlier - are quite a few years older than the post-Standard cars on the accident data files. A major portion of the analysis was devoted to identifying and removing the resulting biases. Several independent tests which were performed an the data files appear to suggest that the age biases and other confounding factors may have been successfully removed.
The missing data rate on injury-causing contact points was high (30 percent) in the National Crash Severity Study and it varied from one team to another. It was necessary to devise analytic techniques for removing the consequent biases. The NCSS file did not contain information on steering column compression, thereby precluding a rigorous statistical comparison of injury severity and column compression.
In general, though, the findings and conclusions of this evaluation may be viewed with confidence because of the harmony between the statistical results, in-depth findings and engineering intuition. Earlier studies of Standards 203 and 204 were largely consistent with the NCSS, FARS, and MDAI analyses performed for this evaluation. Many of the principal findings were supported by two or more independent analysis procedures or data sources.
The principal findings and conclusions of the study are the following:
Effectiveness and benefits of Standards 203 and 204 - fatalities
Effectiveness and benefits of Standards 203 and 204 - serious injuries
Cost of Standards 203 and 204
|Modifications needed for meeting the Standards||
|Voluntary steering wheel improvements||
|Fuel consumption due to 1.11 pound weight increase||
|(in 1978 dollars)|
Displacement of the steering column into the passenger compartment
Why have Standards 203 and 204 been effective?
Shortcomings of Standards 203 and 204
Side effects of Standards 203 and 204
Comparison of alternative energy absorbing devices
Potential for improving Standards 203 and 204