The subject of this report is a study of the problem of external safety of windshields under impact from flying stones. Laminated safety glass is resistant to stone impact;​ tempered glass (for which no road experience is available in the U.S. because it is not used) can only be classified as offering limited safety. More than 1000 road driving trials were made to determine some of the limits of safety offered by the various types of "safety" glass:​ 1. tempered glass;​ 2. chemically-strengthened glass;​ 3. laminated construction with chemically-strengthened glass;​ 4. laminated safety glass with different thicknesses of the interlayer.

It was found that windshields made of tempered glass could suddenly break during the study, and this can be considered a potential cause of an accident, The visibility along the stopping distance is not guaranteed. This also applies to laminates made with strengthened glass -- a development which is being pressed in the United States. Such strengthened constructions are not penetrated by stones. However, the risk involved in covering the stopping distance with limited visibility is increased, since some drivers in the face of this danger who have no "second of fear" (delayed reaction) could punch out a hole in an overall-crazed windshield when it is made of a single piece of strengthened glass.