An Evaluation
of Windshield Glazing
and Installation Methods for Passenger CarsCharles J. Kahane, Ph.D.
Abstract
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:
- HPR glazing doubled the impact velocity needed for the
occupant's head to penetrate the windshield, preventing 39,000 serious lacerations and
8,000 facial fractures per year.
- Adhesive bonding halved windshield bond separation and
occupant ejection through the windshield portal, saving 105 lives per year.
- HPR glazing did not increase the risk of concussions and
adhesive bonding did not increase the injuries of persons who were not ejected.
- HPR glazing added $6 (in 1982 dollars) to the lifetime cost of
owning and operating a car, but adhesive bonding saved $15 per car.
Summary
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:
Principal Findings
Effect of HPR glazing on windshield penetration by
occupants
- When an occupant's head strikes a safety-glass windshield and
tears and penetrates the plastic interlayer, the risk of serious lacerations or fractures
to the face, scalp, eyes, nose or mouth is 3 times greater than when the impact merely
breaks the glass but leaves the plastic layer intact.
- HPR glazing reduced the likelihood of an occupant penetrating
the windshield in frontal crashes by 78 percent, relative to pre-HPR glazing.
- With pre-HPR glazing, there was a 50 percent probability that
an unbelted occupant would penetrate the windshield in a frontal crash with a Delta V of
14 miles per hour. With HPR glazing, the likelihood of penetration does not reach 50
percent until Delta V is 31 miles per hour.
Injury-reducing effectiveness of HPR
Effect of windshield installation method on windshield
retention in crashes
Effect of windshield installation method on occupant
ejection
- In American cars whose windshields were installed by rubber
gaskets, 15 percent of the occupant ejections (complete or partial) took place through the
windshield portal: in pre-Standard 212 Volkswagens, 17 percent. (Persons who merely
penetrate the windshield's plastic interlayer with part of their heads are not
normally coded as "partially ejected" by NHTSA accident investigators.)
- Adhesive bonding reduced the risk of ejection through the
windshield portal by 50 percent in American cars (confidence bounds: 34 to 66 percent).
The clips installed in Volkswagens in response to Standard 212 had the same effect.
Reductions of complete and partial ejection were similar.
- On the other hand, only 30 percent of the persons who were
ejected through the windshield portal received their most serious injuries as a
consequence of the ejection--i.e., from objects exterior to the passenger compartment. As
a result, adhesive bonding saves 15 percent (50% of 30%) of the deaths and serious
injuries of windshield ejectees (confidence bounds: 7 to 22 percent). Standard 212 had the
same effect in Volkswagens.
Effect of windshield installation method on windshield
penetration by occupants
- There was no evidence that tighter bonding increased the risk
of an occupant penetrating the windshield. In fact, the following nonsignificant
reductions were observed:
| 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
Cost of HPR glazing
- The incremental costs per car (in 1982 dollars) for HPR
glazing, relative to pre-HPR, based on analyses of vehicle components, are the following:
| Initial purchase price reduction |
$ 4.45 |
| Lifetime fuel savings due to 1.05 pound weight reduction |
1.05 |
| TOTAL SAVINGS PER CAR |
$ 5.50 |
- The annual cost of HPR glazing in the United States (based on
10 million cars sold) is $55 million.
Cost savings due to adhesive bonding
- Adhesive bonding was a less costly way to install a windshield
than rubber gaskets. The cost savings per car (in 1982 dollars), based on analyses of
vehicle components, are the following:
| Initial purchase price reduction |
$11.50 |
| Lifetime fuel savings due to 3.98 pound weight reduction |
3.98 |
| TOTAL SAVES PER CAR |
$15.48 |
- The annual savings due to adhesive bonding in the United
States (based on 7.5 million domestic cars sold) is $116 million.
Annual benefits of HPR glazing
- The annual benefits, when all cars in the United States have
HPR glazing, will be:
| 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
- When all domestic cars in the United States will have
windshields installed by adhesive bonding, it will save 105 lives per year (confidence
bounds: 35 to 175) and 160 nonfatal AIS 3-5 (serious) casualties.
Annual benefits of Standard 212 for Volkswagens
- When all Volkswagens registered in the United States meet
Standard 212, it will save 7 lives and 11 AIS 3-5 casualties per year.
Cost-effectiveness
- Since HPR windshields save 47,000 AIS 2-4 injuries and cost
$55 million, they eliminate 850 AIS 2-4 injuries per million dollars of cost (confidence
bounds: 560 to 1130).
- Adhesive bonding saves lives while reducing the cost of a car.
Conclusions
HPR Windshields
- HPR glazing greatly reduced the risk of serious lacerations of
the face, scalp and mouth, fractures of the facial bones and nose and occular avulsions.
- The HPR windshield achieved its objective of steeply
increasing the impact velocity needed for an occupant's head to tear and penetrate through
the windshield's plastic interlayer. That explains HPR's success in mitigating the types
of serious injuries listed above, because all of them are characteristically associated
with penetrated windshields.
- The penetration velocities of windshields, both HPR and
pre-HPR, in highway accidents were almost identical to those observed in laboratory tests.
In short, HPR fully delivered in real crashes what it promised in the laboratory.
- Also, as predicted from laboratory testing, HPR had no
negative side effects, such as increasing the risk of injuries associated with blunt
impact trauma (concussions, contusions and complaints of pain). It had little or no effect
on fatalities.
- The accident data indicate that HPR has also reduced minor
lacerations significantly. Those injuries are typically associated with windshields that
are cracked but penetrated. Their reduction confirms heretofore anecdotal evidence that
the interlayer, in addition to resisting penetration, causes glass to crack into smaller,
less injurious pieces. Nevertheless, the majority of minor lacerations still remains even
after HPR.
- About half of the much smaller number of serious injuries
which still occur after HPR are concussions--blunt impact trauma. The other half are
lacerations and fractures: two-thirds of them did not involve windshield penetration and
only the remaining third occurred at speeds too high for HPR to prevent penetration. In
other words, what was once the most characteristic windshield-related serious injury has
been largely eliminated.
- HPR glazing is a highly cost-effective safety device. HPR
eliminated about 80 percent of penetration-related serious lacerations. No other safety
device evaluated by NHTSA to date (October 1984) has come that close to eliminating the
injury mechanism it was targeted to mitigate.
Windshield installation methods
- The risk or (complete or partial) occupant ejection through
the windshield portal was significantly reduced when domestic manufacturers began to
install windshields by adhesive bonding rather than rubber gaskets. A similar reduction
was accomplished when Volkswagen began to clip its rubber gaskets to the car's windshield
frame, in response to Standard 212.
- Unlike the situation that prevails with other ejection
portals, the majority of persons ejected through the windshield portal received their most
serious injuries before they left the passenger compartment--and would still have received
them even if their ejection had been prevented. This attenuates the life-saving potential
of tighter windshield installation methods; nevertheless adhesive bonding and Volkswagen's
clipping of the gasket significantly reduced fatalities and serious injuries.
- Virtually all ejections through the windshield portal occur
after the windshield has been partially or completely dislodged from its frame. Adhesive
bonding and Volkswagen's clipping prevented ejection because they reduced the amount of
windshield separation from the frame--in fact, the reductions in ejection and bond
separation were nearly identical.
- Butyl tape and polyurethane sealant--two alternative adhesive
bonding techniques--provide approximately equal windshield retention in crashes.
- The types of rubber gasket installations found in American
cars could have or did pass Standard 212. The domestic manufacturer's shift from rubber
gaskets to adhesive bonding was not necessitated by Standard 212 but was prompted by other
factors, such additional safety or cost savings. By contrast, the rubber gaskets of
pre-Standard 212 Volkswagens were much closer to a "pop-out" design; the gaskets
were clipped to the frame at the time that Standard 212 took effect. Even the
post-standard Volkswagen windshields were more loosely installed than those of
pre-standard American cars.
- In cars with HPR windshields, the shift to adhesive bonding
clearly did not have any negative side effects such as reducing windshield penetration
velocity or increasing the risk of any type of injury to occupants who are not ejected.
Neither did the shift to clipped rubber gaskets in Volkswagens.
- The accident data strongly confirm Patrick's and Trosien's
sled tests with dummies and full windshield assemblies, which also showed no side effects.
They do not support Rodloff and Breitenbuerger's drop tests of headforms onto
less-than-fullsize glazing samples, which showed that tight bonding defeated a substantial
proportion of HPR's gain in penetration velocity. It must be concluded that the drop tests
simulated the interaction of the windshield and the frame in crashes less realistically
than the sled tests.
- The accident data of this report show that Fargo's analysis of
ACIR accident data, which was based on pre-HPR windshields and found significant negative
side effects for adhesive bonding, cannot be carried over to HPR windshields.
- In cars with pre-HPR windshields, one of the accident files
analyzed in this report associated significant increases in lacerations, concussions and
minor blunt-impact trauma with adhesive bonding, supporting Fargo's results. But analyses
of two other accident files did not confirm that association. The question or whether
adhesive bonding had negative side effects in pre-HPR cars remains unresolved; it has,
however, become moot because few pre-1966 cars remain on the road.
- The shift from rubber gaskets to adhesive bonding reduced the
cost of purchasing and operating a car.
- Since adhesive bonding (in cars with HPR windshields) provided
significant benefits without negative side effects while reducing cost, it is certainly a
cost-effective safety improvement.
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