Safety glass (thing)

Safety glass is a relatively mature feature in automotive history. The idea dates back to 1905, when Englishman John Crew Wood first patented laminated safety glass. Originally, tree resin was used to laminate the glass layers, and later, gelatin was used in safety glass products during the first world war. In 1927, knowing that glass injuries during otherwise minor car wrecks were shown to occur in a significant number of instances, automakers began manufacturing cars incorporating safety glass". There are two main forms of safety glass:

• Laminated glass - modern safety glass is formed by laminating a thin clear film of flexible plastic called polyvinyl butyral, or PVB; and is used mainly in the windshield of the car. When the glass is broken, the film adheres to the glass, preventing the pieces from flying out and causing cuts and injuries. Since the laminate is difficult to penetrate, it also serves to hold occupants in the vehicle during an accident, and thus prevent injuries from Premature Ejection. Banks use multi-layered laminated glass to protect tellers from robbery attempts, and a similar lamination process using glass and polycarbonate is used for bulletproof glass. Other benefits include the ability to block high frequency sound and about 97% of ultraviolet light (thus preventing sunburns on road trips!)
  

• Tempered glass - formed by a special tempering process that heats the glass, then cools it quickly to harden it. Tempered glass is five to ten times as strong as regular glass, and is used in side and rear windows of cars. Because of the tempering process, the glass breaks into small cubical pieces without sharp edges rather than jagged shards. If you wear polarized sunglasses, you will usually see a spectral pattern in tempered glass on sunny days. Tempered glass is also used in monitors, oven and storm doors, refrigerator shelves, skylights, and LCD's.

Windshield glass replacement is nearly always covered by auto insurance, usually with a reasonable copayment (often $100 in the US). Many companies also reimburse for repair or replacement of cracked windshield depending upon the degree of damage. Overall, it is safe to say that safety glass provides a clear advantage for drivers.

www.edmunds.com/ownership/safety/articles/43816/article.html
www.howstuffworks.com/question508.htm