An Evaluation of the Effects of Glass-Plastic Windshield Glazing in Passenger Cars

Glenn G. Parsons

Abstract

Following revision of the applicable Federal safety standard in 1983, two motor vehicle manufacturers equipped some of their cars with glass-plastic windshields for testing in rental fleets. One company also installed the windshield in regular production cars for a brief period. The windshield was thought to have high potential for reducing windshield-caused lacerations to occupants involved in crashes. There were also concerns over the durability of the product. This study is an evaluation of the safety, durability, and cost of glass-plastic windshield glazing. It is based on analyses of data from State crash files, fleet tests, and other sources. The study findings are:

• Safety. Crash data indicate that the injury reduction potential of glass-plastic windshields is substantially less than predicted.
• Durability. Fleet and warranty claim data indicate that durability problems are greater than anticipated.
• Costs. A glass-plastic windshield adds $65 to the cost of a new car. Additional "durability" costs would also accrue. Replacement cost is estimated to exceed $1,700.
• Today's high rates of safety belt use, coupled with the growing number of air bag-equipped cars, mean that windshield-caused injuries have decreased and will continue to decline.

Introduction

Under Executive Order 12866 (and prior Executive Order 12291), the National Highway Traffic Safety Administration (NHTSA) has been directed to carry out periodic reviews of the automotive safety regulations which it has issued. These reviews provide a means of measuring the impacts of those regulations, in terms of the benefits which accrue to, and the costs which are imposed upon, the American public.

This study is a review of the effects of glass-plastic windshield glazing. It is NHTSA's second review of the effects of windshield glazing in passenger cars. The first study, published in 1985, evaluated the safety benefits and costs of conventional windshield glazing which has been standard equipment in American-made vehicles since 1966. The conventional windshield, often referred to as the "HPR" (or High Penetration Resistant) windshield, was found to be a major safety improvement over previous glazing designs, and was credited with bringing about a major reduction in the frequency and severity of head and facial injuries which resulted from occupants being thrown against the windshield in crashes. The primary benefit of the HPR design was a large reduction in the more severe facial lacerations and fractures, with a more modest reduction in less severe, or minor lacerations. These benefits were attributed to improved production techniques which significantly increased the penetration resistance of the windshield. Occupant penetration of the windshield is generally associated with more severe injury.

Federal Motor Vehicle Safety Standard (FMVSS) No. 205, issued by NHTSA in January 1968, prescribes safety requirements for all glazing materials used in motor vehicles, including the windshield, the windows, and any interior partitions. The purpose of this standard is to: (1) reduce injuries resulting from impact with the glazing surfaces; (2) minimize the possibility of occupants being thrown through the windshield in collisions; and (3) ensure a necessary degree of transparency in the glazing for driver visibility. In 1983, the agency amended FMVSS No. 205 to permit the use of a new type of glazing, known as "glass-plastic" glazing. This change to the standard did not require the use of glass-plastic glazing, but rather permitted the use of this material at the option of the motor vehicle manufacturer.

The essential promising aspect of the glass-plastic windshield was its considered potential to further reduce windshield-induced lacerations. While the HPR windshield had substantially reduced these types of injuries, a considerable number still remained, primarily those in the minor severity category. The conventional windshield is a three-ply design consisting of two plies of glass sandwiched around a thin interply of plastic (polyvinylbutyral). The glass- plastic windshield is of similar design with the exception of an inner plastic liner (polyurethane) that is bonded to the inside glass ply -- i.e., the side of the windshield which faces the passenger compartment. The inner plastic liner would be expected to provide additional protection (over that afforded by the HPR design) against cuts from broken glass shards produced when occupants collide with the windshield during crashes. The plastic liner would provide a "containment mechanism" for the broken pieces of glass, thereby reducing the occupant's chances of coming into direct contact with the sharp edges of these glass fragments. While glass-plastic windshields were expected to substantially reduce lacerative injuries, there was concern that the inner plastic layer, being a much softer material than glass, could be susceptible to damage that could degrade driver visibility and reduce windshield durability. There was also some concern that the stiffer surface presented by the 4-ply glass-plastic windshield might contribute to a greater incidence of blunt impact injuries (i.e., concussions, contusions, complaint of pain). Overall, however, the potential safety gain from glass-plastic glazing was believed to far outweigh possible durability and other problems, and, therefore, the agency elected to permit (but not require) its use in order that real-world data might be developed to provide an evaluation of these issues.

Glass-plastic glazing was first developed in France by the Saint Gobain Vitrage Company, and some of the European car companies (among them, Peugeot, Porsche, and Mercedes) had fitted a limited number of their vehicles with glass-plastic windshields in the late 1970's and early 1980's to test the material in the market place. In the early 1980's, two domestic companies, General Motors and Ford, equipped a number of their vehicles with glass-plastic windshields and placed them in rental fleets to field test the windshields. Later, in 1984, General Motors introduced the glass-plastic windshield to the general public, by making it standard equipment on one of its luxury car models, the Cadillac Seville Elegante. Early in the 1985 model year, the windshield was made standard on all Seville models, and for model years 1986 and 1987, the company expanded its use of the plastic windshield glazing, making it standard on all Cadillac Eldorados, all Buick Rivieras, all Oldsmobile Toronados, and all Pontiac 6000 STE'S. At the end of the 1987 model year, however, General Motors discontinued all use of glass-plastic windshields in its regular production vehicles. GM stated that the reason for discontinuing installation of the windshield ws because of its high replacement costs for customers and high warranty costs for the company. It is estimated that approximately 210,000 regular production GM cars with glass-plastic windshields were produced before the company halted use of the windshield. No other car companies, domestic or import, have since equipped any of their U. S. marketed regular production vehicles with glass-plastic glazing.

Study Objectives and Data Sources

The objectives of this study are threefold:

(1) to estimate the extent to which glass-plastic windshield glazing could reduce lacerative injuries resulting from occupant contact with the windshield in crashes,

(2) to assess the nature, extent, and consequence of durability problems that could be experienced if glass-plastic windshields were installed in vehicles on a volume basis, and

(3) to estimate the costs that would be incurred from the use of glass-plastic windshields in motor vehicles.

The injury reduction effect of glass-plastic windshields is primarily based on the analysis of police reported crash data from three States, New York, Pennsylvania, and Indiana. State data -- in particular data from New York State -- constituted a major source of the information utilized in NHTSA's earlier study of the effects of HPR windshield glazing. Although not without certain limitations, these States provide detailed injury data in their automated crash files, such as type of injury and body location of injury, which are necessary to estimate the effect of the windshield glazing. The analyses are based on comparing the rate of bleeding injuries (primarily to the head and face) sustained by front seat occupants of glass-plastic vehicles (1985, 1986, and 1987 GM vehicles) with the rates of similar types of injuries sustained by front seat occupants of a control sample of vehicles. Bleeding injuries are considered to be generally synonymous with lacerative injuries. The control vehicles are GM cars of similar size, weight, and price range, which are equipped with conventional (i.e., HPR) windshields. The crash and injury results of special fleet tests of vehicles equipped with glass-plastic windshields are also reviewed.

The durability information is based primarily on the GM and Ford rental fleet field tests, together with other information, including warranty data, obtained from these manufacturers. The costs of glass-plastic glazing are based on earlier estimates made by the agency, on information solicited from the motor vehicle manufacturers, and on data on the aftermarket replacement costs of the glazing. A consumer survey of vehicle owners had originally been planned to obtain "first-hand" information on owner experience with the windshields, but budgetary priorities precluded the agency from conducting the survey.

It should be noted that major advances in motor vehicle safety have occurred since the agency amended FMVSS 205 in 1983 to permit the use of glass-plastic glazing. Safety belt use has increased to levels of over 60 percent. compared to a low 14 percent in 1983. Also, air bags not only have received wide acceptance, but will be required in all new cars and light trucks by the late 1990's. These significant increases in occupant protection mean that the numbers of occupants who impact the windshield (thereby being exposed to lacerative injuries from broken glass) have been substantially reduced and will continue to diminish. Hence, the magnitude of the safety problem originally targeted by glass-plastic windshield glazing is being substantially reduced. This situation contrasts rather sharply with the occupant restraint picture in 1983 when NHTSA amended Standard 205. At that time, the agency recognized that substantial changes in the availability and use of passive restraints could alter the need for the additional occupant protection qualities expected from glass- plastic glazing. However, if automatic restraints were eventually installed in passenger cars, the agency believed that they would primarily be passive belt designs (as opposed to air bag systems) and that a substantial portion of motorists would elect not to use them. This was also prior to the period of State Mandatory Restraint Use Laws and other developments which have since fostered marked increases in the use of safety belt systems, both active and passive.

Following are the principal findings and conclusions of this study:

Injury Reduction

• The analysis of available crash data from three States found no reduction in bleeding (or lacerative) injuries for vehicles equipped with glass-plastic windshields as compared to vehicles equipped with conventional windshields. Small sample sizes of injury data plus other restrictions in the data serve to limit the reliability of this finding, and the results are not interpreted as precluding that glass-plastic windshields may have a beneficial effect in reducing lacerative injuries.
  
  

• Data from fleet operations were insufficient for analyses with respect to the injury reduction potential of glass-plastic glazing. However, these data did provide limited evidence that lacerations do indeed result from glass-plastic glazing. lacerations from glass-plastic glazing were described as "skin-splitting", or "tearing-type" injuries, resulting from blunt impact with the plastic inner liner of the windshield, rather than lacerations from broken glass. In these injury cases, the windshield glass was broken, but the broken pieces did not penetrate through the plastic inner liner.

Durability

• Although not necessarily representative of the motoring public in general, data from rental fleet operations indicates that durability issues associated with glass-plastic windshields could be of significant concern. Primarily, these issues revolve around the susceptibility of the plastic inner liner to damage (i.e., cuts, scratches) from the everyday operating environment:
  

  - in one fleet, an estimated 42 percent of the windshields were found to have moderate-to-heavy scratches and cuts after 15 months of operation. The vehicle manufacturer was concerned that this damage might not be acceptable to longer term owners.
  

  - in another fleet, 44 to 48 percent of the windshields were replaced due to damage occurring during fleet operations, and in order to "prepare the vehicles for resale" at the end of the one to one and one-half year test period. Available data indicate a replacement rate of 10 to 15 percent for conventional windshields in comparable rental fleet service.
  
  

  - all glass-plastic windshields replaced in the rental fleets were replaced with conventional windshields.
  
  

• Warranty claim data from the one manufacturer which briefly introduced glass-plastic windshield glazing to the general public also indicated that durability of the material was a concern. Warranty claim rates of 12.6 percent, based on 12 months/ 12, 000 miles of service, were more than four times as high as for conventional windshields. Vehicle owners cited poor visibility and scratching/scoring as the primary reasons for windshield replacement.

Costs

• It is estimated that a glass-plastic windshield, if installed in volume quantities, would add $65 to the cost of a new car. Additional consumer costs would be expected due to damaged windshields that would have to be replaced after the new vehicle warranty period expired, or for damage not covered under the warranty.
  
  
• Data on aftermarket windshield replacement show the cost of replacing a glass-plastic windshield to be over $1,700, or more than $1,200 above the cost of replacing a conventional windshield. To what extent this high cost is due to limited production quantities or to glass replacement shop inexperience/unfamiliarity with the glass-plastic glazing, as opposed to lower durability characteristics of the glass-plastic windshield, cannot be answered with available data. Due to this very high cost, it is believed that almost all aftermarket replacements of glass-plastic windshields have been made with conventional windshields rather than the original equipment glass-plastic windshields.
  
  
• Both motor vehicle and glass manufacturers cited high costs as the reason why the market for glass-plastic windshields failed to develop.
  

• Analyses of available crash information, because of small sample sizes and other limitations in the data, are insufficient to support firm conclusions. Nevertheless, the analyses of State crash data, together with limited evidence from fleet crash data, do suggest that the actual lacerative injury reduction benefits from glass-plastic lazing are likely to be substantially less than the essential elimination of windshield-caused lacerations, as originally projected by the agency. While there is evidence that the inner plastic liner does serve to reduce the occupant's chances of coming into contact with broken glass shards, lacerations can still occur from blunt impact with the inner liner. Data are insufficient to determine whether these injuries differ from the lacerations due to conventional windshields, with respect to severity or facial scarring potential.
  
  
• Data from rental fleet operations and manufacturer warranty claims indicate that durability problems are greater than originally anticipated by the agency. Potential solutions to these durability concerns, including windshield warning labels and "special care" information in the vehicle owners manual, did not appear sufficient to prevent damage from occurring to the plastic inner liner, nor warranty claim rates several times higher than for conventional windshields. The high replacement cost of glass-plastic windshields is also believed to reflect, at least partially, the lower durability characteristics of the glazing.
  
  
• Depending on the nature and extent, damage to the plastic inner liner of the glass-plastic windshield could degrade driver visibility. thereby creating a safety concern. Insufficient data are available to quantify the significance of this issue. Also, the indicated widespread replacement of damaged glass-plastic windshields with conventional windshields, due to cost or other considerations, has the effect of negating any safety benefits inherent in the glass-plastic glazing.
  
  
• The higher component and durability costs of the glass-plastic windshield, compared to that for a conventional windshield, would appear to be high relative to the injury reduction benefits that might be realized from the glazing.
  
  
• Today's high rates of safety belt use, together with the universal acceptance of air bags - in contrast to the situation in 1983 when the agency permitted the use of glass-plastic glazing - mean that the size of the lacerative injury problem due to occupant-windshield contact is substantially smaller than it was a decade ago and will continue to diminish.