|NHTSA Report Number DOT HS 806 693||February 1985|
Charles J. Kahane, Ph.D.
Federal Motor Vehicle Safety Standard 205 sets requirements for the penetration resistance of windshields upon impact while Standard 212 regulates windshield retention in crashes. The High Penetration Resistant (HPR) windshield, developed during the 1960's, was designed to crumple and deform at speeds where earlier windshields would have been penetrated by head impacts. Adhesive bonding provided a tighter windshield installation method than earlier rubber gasket designs. The objectives of this agency staff evaluation are to determine if HPR windshields and adhesive bonding achieved their performance objectives in highway crashes and to measure their casualty-reducing benefits, side effects and costs. The study is based on statistical analyses of National Crash Severity Study, New York, Texas, mu1tidiscip1inary Accident Investigation and Fatal Accident Reporting System data.
It was found that:
Since 1960, there have been major changes in the design of windshields for passenger cars and in the techniques whereby windshields are installed in cars.
In 1965, the domestic manufacturers installed High Penetration Resistant (HPR) windshields, on an experimental basis, in a few models and in 1966, HPR became standard equipment in all domestic cars. Before HPR, the plastic interlayer of safety glazing used in windshields was easily torn by broken glass, permitting the occupant1s head to tear through and penetrate the windshield in low-speed crashes. Windshield penetration was believed to be the cause of most of the disfiguring or disabling head injuries associated with windshield contact. Rodloff, Patrick, Rieser and other researchers found techniques to obtain a looser glass-plastic bond in safety glazing, allowing the glass to crumple away rather than tear the plastic. The new manufacturing techniques, in combination with a thicker plastic layer, became known as the "HPR windshield", which was found to double the speed needed for the windshield to be penetrated in laboratory impact tests. Federal Motor Vehicle Safety Standard 205, which took effect on January 1, 1968, incorporated American National Standards Institute's safety codes which the motor vehicle industry had already imposed upon themselves to assure that all motor vehicles have windshields as penetration-resistant as HPR.
Before 1963, windshields were installed in a car by means of a rubber gasket. In 1963, butyl tape was used to adhesively bond the windshield to the frame on a small test fleet of General Motors cars. Adhesive bonding became standard on a few GM models in 1964. The domestic manufacturers gradually shifted from rubber gaskets to adhesive bonding (initially butyl tape and later, in some cases, polyurethane sealant) after 1964, but rubber gaskets remained on some domestic models until 1978. The objectives of adhesive bonding were not explicitly stated, but two may be inferred: to provide a tighter bond between windshield and car, preventing the windshield from becoming dislodged in a crash, denying occupants an avenue for ejection through the gap between windshield and frame; to reduce manufacturing cost by eliminating the rubber gasket. On January 1, 1970, in the middle of the transition from rubber gaskets to adhesive bonding, Federal Motor Vehicle Safety Standard 212 took effect for passenger cars. Standard 212 limits the amount of windshield bond separation allowed in a 30 mph barrier crash and has the explicit objective of preventing occupant ejection through the windshield portal. But the relationship of Standard 212 to adhesive bonding is not clear, since, as stated above, rubber gaskets continued to be used in some models well after 1970. It is possible that a 1976 modification in the temperature range for Standard 212 testing may also have accelerated the shift to adhesive bonding.
Foreign cars, as a matter or fact, continued to use mostly rubber gaskets throughout the 1970's. But Volkswagen, which had virtually a "pop-out" windshield before 1970, did install clips between the gasket and the frame in response to Standard 212. It is possible that other German manufacturers also implemented similar devices about that time.
Executive Order 12291 (February 1981) requires agencies to evaluate their existing major regulations, including any rule whose annual effect on the economy is $100 million or more. 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 and to assess cost-effectiveness.
This report is an evaluation of HPR windshields for passenger cars, adhesive bonding of the windshields of domestic cars and the changes in the installation of Volkswagen and other German windshields made in response to Standard 212. (HPR glazing and adhesive bonding were also implemented in vehicles other than passenger cars, but that will be evaluated at a later date.)
The report does not evaluate the effects of the shift from laminated to tempered side windows which took place in about 1960--there were far too few cases on NHTSA accident files of occupants who were injured by contact with side windows in cars of the 1960 era. It also does not evaluate glass-plastic glazing concepts such as Securiflex because they have not yet (October 1984) been implemented in large numbers on cars sold in the United States. NHTSA evaluations of existing safety devices, as stated above, are based on the actual operating experience of production vehicles: something not yet available in sufficient quantity for glass-plastic glazing. It should be noted however, that laboratory tests show that glass-plastic glazing may have great potential for reducing minor facial lacerations (a great many of which still remain, even after HPR) and occupant ejection through side windows. If the concept is implemented on a large number of production vehicles, NHTSA will certainly evaluate their on-the-road experience.
HPR windshields have already been informally evaluated. The dramatic reduction in the demand for facial plastic surgery following the introduction of HPR made it clear to the safety community that HPR has been, perhaps, more successful than any other standard. The effectiveness of HPR has been shown in a number of laboratory studies and statistical accident analyses. It remains for this evaluation to give specific estimates of the numbers and types of injuries prevented by HPR, to compare laboratory and highway accident performance, and to investigate the possibility of negative side effects such as blunt impact trauma or secondary benefits such as a reduction of minor injuries.
Windshield installation methods, by contrast, have been a controversial subject since the mid-1960's. On the one hand, no study to date appears to have shown whether or not tighter bonding methods achieved Standard 212's goal of reducing occupant ejection. On the other hand, studies by Fargo (accident analysis of pre-HPR cars) and Rodloff and Breitenbuerger (drop tests with HPR glazing) warn that tight bonding has serious negative side effects for persons who are not ejected: lacerations, blunt impact trauma and a reduced windshield penetration velocity. But Patrick's and Trosien's sled tests with dummies found little or no side effect. Who is right? Thus, the evaluation must analyze the effect of installation method on ejection and on persons who are not ejected. Both analyses must be performed separately for domestic cars (effect of adhesive bonding) and German cars (effect of Standard 212); the analysis of persons who are not ejected, separately for pre-HPR and post-HPR cars.
The strategy of this evaluation was to perform parallel statistical "injury" and "engineering" analyses of accident data. In the case of HPR, the "injury" analysis of the reduction of various types of head trauma was paralleled by an "engineering" analysis of the velocities at which heads penetrate windshields in highway accidents. The objectives were to give an engineering explanation of why injuries were reduced and to compare hardware performance in accidents to the laboratory. In the study of the effect of windshield installation method on ejection, the "injury" analysis of occupant ejection rates was accompanied by an "engineering" analysis of windshield retention in crashes. The analysis of the side effects of windshield installation method on occupants who were not ejected again compared types of head injuries and penetration velocities.
The "engineering" analyses were based on National Crash Severity Study (NCSS) data. The "injury" analyses, in each case, were based on at least 3 files: effect of HPR on injury rates--NCSS, New York State, Texas and Fatal Accident Reporting System (FARS); effect of installation method on ejection--NC55, Multidisciplinary Accident Investigation (MOAI) and FARS; effect of installation method on injury rates--New York State, NCSS and Texas. New York data were especially useful because they identified the body region and type of injury over a large sample of accidents. When large data files were used (FARS, New York, Texas, the analysis of German cars was limited to Volkswagen, where it is relatively clear that clips were installed very close to the beginning of the 1970 model year. For the smaller data files (NCSS, MDAI), the other German makes are included to increase the available sample size, even though it is not as well known when and if similar modifications were made. Thus, throughout the report, results on "German cars" are the ones based on NCSS and/or MDAI; those on "Volkswagen" are based on the other files. Practically speaking, though, the distinction is of minor importance since Volkswagen accounted for over 85 percent of the German cars sold here during 1965-74.
The cost of the vehicle modifications was estimated by analyzing the components of vehicles produced before and after the modification.
The most important conclusions of this evaluation are that HPR glazing dramatically reduced the number and severity of facial lacerations and fractures while doubling the impact velocity needed to penetrate the windshield in crashes. Adhesive bonding saved lives because it halved windshield separation in crashes and occupant ejection through the windshield portal; the clips installed in the rubber gaskets of Volkswagens in response to Standard 212 had the same effect. In cars with HPR windshields, the installation method had little or no side effect on the injuries of persons who were not ejected. Because each of these conclusions is supported by analyses of multiple data files, which are remarkably consistent with one another and with the "engineering" analyses, they may be stated confidently.
In two areas, conclusions are drawn less firmly. One concerns the proportion of ejectees through the windshield portal who were killed by injuries sustained while they were still inside the passenger compartment. This proportion is estimated with reasonable precision from NCSS and MDAI data. It is then assumed to equal the proportion of persons, saved from ejection by adhesive bonding, who would have died anyway from interior contacts. The FARS data, unfortunately, were unsuited for an independent verification of this plausible assumption. Thus, in this case, "evidence from multiple data files" is lacking. The other area is the effect of adhesive bonding on injury risk in cars with pre-HPR windshields. New York data show significant negative effects but NCSS and Texas data show none. Thus, while the evaluation clearly shows no side effects with today's windshields, it is unable to resolve the controversy about adhesive bonding in cars with pre-HPR windshields--fortunately, the question has become moot because so few of them remain on the road.
The principal findings and conclusions of the study are the following:
Effect of HPR glazing on windshield penetration by occupants
Injury-reducing effectiveness of HPR
Reduction for HPR (%)
|Best Estimate||Confidence Bounds|
|AIS 2-4 lacerations||74||65 to 83|
|AIS 2-4 eye, nose or mouth injuries||72||58 to 86|
|AIS 2-4 fractures||56||27 to 85|
Those are the types of injuries most characteristically associated with penetration of the windshield.
Effect of windshield installation method on windshield retention in crashes
|American cars - rubber gaskets||22|
|American cars - adhesive bonding||15|
|German cars - pre-Standard 212 gaskets||59|
|German cars - post-Standard 212 gaskets||39|
Standard 212 allows 25 percent bond separation in a staged 30 mph barrier impact.
Effect of windshield installation method on occupant ejection
Effect of windshield installation method on windshield penetration by occupants
|Adhesive bonding vs. rubber gasket in American cars:||1 percent|
|Adhesive bonding vs. rubber gasket in American pre-HPR cars:||7 percent|
|Post-Standard 212 gasket vs. pre-Standard in German HPR cars:||19 percent|
Effect of windshield installation method on the injuries of persons who are not ejected
|Increase for Adhesive Bonding (%)|
(Cars with Pre-HPR Windshields)
|"Severe bleeding" (i.e., nonminor lacerations)||20|
|Contusions and complaints of pain||20|
The Texas data do not show any increase, however, in overall injury rates.
Cost of HPR glazing
|Initial purchase price reduction||$ 4.45|
|Lifetime fuel savings due to 1.05 pound weight reduction||1.05|
|TOTAL SAVINGS PER CAR||$ 5.50|
Cost savings due to adhesive bonding
|Initial purchase price reduction||$11.50|
|Lifetime fuel savings due to 3.98 pound weight reduction||3.98|
|TOTAL SAVES PER CAR||$15.48|
Annual benefits of HPR glazing
|Reduction of Head Injuries with||Best Estimate||Confidence Bounds|
|AIS 2-4 laceration or avulsion||39,000||25,000 - 53,000|
|AIS 2-4 fracture||8,000||1,000 - 18,000|
|AIS 2-4 (any type)||47,000||31,000 - 62,000|
|AIS 2-4 eye, nose or mouth injury||19,000||9,700 - 29,000|
|AIS 1 laceration||142,000||22,000 - 315,000|
Annual benefits of adhesively bonded windshields in American cars
Annual benefits of Standard 212 for Volkswagens
Windshield installation methods