Windscreens and Safety: A Review

Monash University Accident Research Centre - Report #183 - 2001

Full report in .pdf format [985KB]

Authors:  Pronk, N., Fildes, B., Regan, M., Lenné, M., Truedsson, N., & Olsson, T.


This report describes the research outcomes of a review examining the link between windscreen degradation and road safety. Different types of windscreen damage were identified including sudden impact damage such as cracks and "bulls eyes" and gradual degradation resulting from the constant wear of the surface by small particles such as sand and dirt. The effect of gradual degradation on driving safety has been the focus of this project. A review of the international literature identified a number of studies that suggest that windscreen degradation may impair driver perception. Little research was identified that directly examined the link between windscreen degradation and crash involvement. Relevant national and international standards, regulations and guidelines relating to windscreens were summarised, and devices that can measure windscreen degradation were identified. Finally, recommendations were made for the future direction of the project. 

Executive Summary 

Literature review 

Types of Windscreen Visibility Degradation 

Damage to windscreens typically falls into two categories; sudden impact damage and degradation or wear. 

Sudden impact damage includes cracks and "bulls eyes". Only a couple of studies were identified that considered this type of damage. One suggested that this type of damage may be distracting to drivers (though this was not supported by research) and the other concluded that this type of damage seemed to cause less of a visual problem than the multiple fine scratches and chips resulting from gradual wear. 

Windscreens wear and degrade over time. Through use they are continuously bombarded by small particles such as tiny rocks, sand and dirt that wear the surface. Further to this, windscreen wipers can damage the windscreen over time by scratching tiny particles across the surface. 

Another factor that may compound the effects of windscreen degradation is soiling. It was suggested that grease and grime may result in the production of scattered light and further, that the removal of the grease and grime may result in the windscreen being scratched and damaged. 

De-lamination and "milkiness" can also impact negatively upon the quality of windscreens. Delamination occurs when one or both of the layers of the glass separate from the polyvinylbutyrate interlayer. This can cause different optical effects. Milkiness occurs when the polyvinylbutyral layer separates from the layers of glass and starts to return to its original prelaminated state. The windscreen can start to become opaque and cause difficulty for the driver. 

Migration of the plasticiser can occur following exposure to sun. Under this condition the plasticiser can be liberated from the dashboard and form a thin film on the inside surface of the windscreen. This build up can cause problems for drivers including distortion of in coming light. 

Many vehicles have dashboards that are made from plastic that, in certain light, reflects an image from the dashboard onto the windscreen. This reflection may cause distraction to the driver. 

Stray Light 

When light travels along a path without disturbance it is called "useful light". However the light path can be disturbed causing the beam to change in some way. Light that has been disturbed is referred to as "stray light". The intensity of stray light depends on the scattering angle and the intensity of the original beam. 

Stray light can be generated in soiled or surface damaged windscreens. During the daylight the eye is adapted to stronger light with many different wavelengths, therefore the impact of stray light is not very severe at this time. However in the case of suddenly appearing light sources, such as on-coming headlights at night, the light that hits the windscreen can be deflected into the driver's eye by means of wide angle light scattering, causing disturbances in vision and perception. 

Different types of windscreen damage can result in different stray light effects. For example, small chips tend to scatter light with a halo around the light source, while scratches and grooves tend to scatter light perpendicular to the damaged area and add one or two "tails" to the light source. 

The Link between Windscreen Degradation and Safety 

Findings from several studies suggest that stray light can have a negative impact on driver perception. 

A number of field and laboratory studies (some using a driving simulator) were identified that considered the effects of worn windscreens on driver perception. Some of the key findings from these studies included:

  • drivers may take longer to re-adapt their vision following exposure to the stray light effects created through a worn windscreen ("dazzling") ; 
  • detection distances to objects on the road ahead may be reduced when looking through worn windscreens with stray light effects. Further, the contrast of objects on the road ahead may be reduced by stray light and a consequence of this could be a reduction of visibility distances; 
  • while detection distances may be reduced when driving with a worn windscreen, drivers seem to adapt their speed to their abilities. One study found that those with the best visual performances drove the fastest; 
  • drivers do seem to be able to view objects at a distance when looking through a worn windscreen despite previous research suggesting that this could be a problem (the "Mandelbaum" effect); 
  • dirty windscreens seemed to cause drivers to crash twice as often in a driving simulator (as compared to driving with a new windscreen) and when driving with a degraded windscreen, drivers reaction times to a secondary task may be slowed. The authors of this study concluded that driving with a visually degraded windscreen induces fatigue and performance declines more rapidly than when driving with a non-degraded windscreen; and 
  • older participants may find stray light effects ("glare") to be more debilitating compared with younger participants. 

Very few studies were identified that attempted to quantitatively establish a link between windscreen degradation and crash involvement. Only one simulation study examined the relationship between windscreen degradation and crashes. A study conducted recently in New South Wales by the RTA looking at 4000 cars (from 2500 crashes) collected, amongst other things, information relating to the state of the windscreens. The possibility of working in collaboration with the RTA could be explored in the future, to analyse this data in light of the present project.

Regulations and Guidelines for Windscreen Integrity

Vehicle safety requirements are specified at a number of levels. 

The Motor Vehicle Standards Act 1989 

The object of the Motor Vehicle Standards Act 1989 is to ensure uniform vehicle standards are applicable to all road vehicles that are used in the Australian transportation system. Essentially this is the overall governing national guideline relating to vehicle safety in Australia. 

Australian Design Rules for Motor Vehicles and Trailers (3rd Edition) 

The Australian Design Rules (ADRs) are national standards. The Australian Design Rules for Motor Vehicles and Trailers (3rd Edition) form part of the National Standards determined under section 7 of the Motor Vehicle Standards Act (current Act was passed in 1989). 

The ADRs set out the design standards for vehicle safety. All cars that are first supplied to the Australian market are required to comply with these. This includes vehicles manufactured in Australia as well as those imported from overseas. 

The following ADR was identified as being relevant to the present project. 

  • Australian Design Rule 8/00 Safety Glazing Material 
Australian / New Zealand Standards 

The aim of standards is to regulate the industry of concern to maintain a minimum level of safety. The following Australian/New Zealand standards were identified as being relevant to the present project. 

  • AS/NZS 2080:1995 Safety glass for land vehicles 
  • AS/NZS 2366.1:1999 Windscreen repairs: Part 1: Repair procedures 
  • AS/NZS 2366.2:1999 Windscreen repairs: Part 2: Repair systems 
Overseas Standards 

Several overseas standards were identified. 

  • United Nations - European Standard. Uniform Provisions Concerning the Approval of Safety Glazing and Glazing Material - United Nations Agreement Concerning the Adoption of Uniform Conditions of Approval and Reciprocal Recognition of Approval for Motor Vehicle Equipment and Parts, Addendum 42: Regulation No. 43 
  • ANSI/SAE Z26.1-1996. American National Standard for Safety Glazing Materials for Glazing Motor Vehicles and Motor Vehicle Equipment Operating on Land Highways - Safety Standard 
  • American Standard. Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics - 1977. 
  • German Standard. Road Vehicles - Measurement of scattered light of safety glazing materials for vehicle glazing. Part 1: Annular diaphragm method.
  • JIS R 3211:1998. Japanese Industrial Standard. Safety Glazing Materials for Road Vehicles. 
  • JIS R 3212:1998. Japanese Industrial Standard. Test Method of Safety Glazing Materials for Road Vehicles. 
  • BS AU 242:1991. British Standard. Automotive Windscreen Repair. 
  • AGE AU 251. British Standard. Part I - Specification for Performance of Automotive Laminated Windscreen Repair Systems. Part II - Code of Practice on Repair Methodology. 

Australian Roadworthiness guidelines 

Vehicle roadworthiness guidelines define legal requirements to ensure a minimum level of safety for vehicles. They focus on the safety related aspects of vehicles and aim to ensure that parts have not worn or deteriorated to an extent that they have rendered the vehicle unsafe for use. 

The National Road Transport Commission (NRTC) released draft "Roadworthiness Guidelines" in September 1995 as part of their task to develop uniform laws and administrative guidelines for the "safe and efficient operation of road transport in Australia" (p1 of the draft document). 

Several Australian jurisdictions have based their roadworthiness guidelines directly on this document. 

In this section the guidelines used in each State and Territory of Australia were summarised. 

Administrative guidelines: Assessment of defective vehicles (February 1999) 

The NRTC Administrative Guideline for the Assessment of Defective Vehicles was designed "to help enforcement officers determine the significance of vehicle defects relative to the safety risk they present on the road" (Explanatory Notes to the document). 

This document classifies different types of windscreen damage.

Measurement of windscreen degradation 

This section of the report describes and reviews devices and techniques that are currently available to measure windscreen degradation. 

Early and Modern Techniques 

One of the first techniques for measuring the extent of windscreen degradation was a laboratory-based technique (Allen, 1969). It involved taking photographs of the headlights of an oncoming car through the windscreen. To measure the extent of light scatter, layers of translucent plastic polyethylene were layered over the photograph of the scratch pattern until the scratch pattern could barely be seen. The number of layers provided a crude index of the extent of windscreen scratching. 

Modern techniques express the degree of windscreen degradation in terms of the ratio of the intensity of the stray light to the intensity of useful light, as it is believed that this ratio is critical for the driver (Timmermann, 1985b). This ratio is called the reduced luminance coefficient and is also referred to as the Scattered Light Index (SLI). The higher the SLI, the greater the degree of light scattering and hence windscreen degradation. 

The "Hazemeter" (American standard D 1003-61) is a device that was originally developed in the 1960s (and re-approved in the 1970s) for testing for haze and luminous transmittance of transparent plastics. The general design and procedure of this device are described. 

Stray Light 

Derkum (1991a) identified that stray light emanating from different types of windscreen damage (eg: craters, wiper damage, scratches) produces different types of distributions. Derkum also suggested that the effects produced by different types of damage could have differing effects on driver perception. Further, as windscreen damage may not be uniform across the windscreen, it may be important to consider the location of the damage. This author suggested that it is important to consider both the specific type of damage in addition to the stray light mean value. 

The Stray Light Index as a Measure of Windscreen Degradation 

Two portable windscreen measurement devices were identified. 

A device to measure stray light effects developed by Schwahn Systems, the "StrayLizer", was identified and the measurement technique described. 

Another device, the "IRIS 504SC", developed by DMO Optical Solutions was also identified and described. 

Aviation Measures 

Haze has been measured in aircraft transparencies using a Hazemeter. A portable device, the "Haze-o-Meter II", developed by Armstrong Laboratories for the United States Air Force, was also identified however little information is available regarding this device. 

Where to from here? 

Finally, a research strategy was outlined that could further identify the extent of the windscreen/safety problem and also identify means for improving windscreen safety. This strategy includes defining the extent of the problem (using a crash study and performance study), developing a suitable measurement device, and the development of a suitable standard. 

Sponsoring Organisations: Federal Office of Road Safety; Perfect Glass; Pilkington; VicRoads; Windscreens O'Brien