Safety glass

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

Although Englishman John Crew Wood patented a form of laminated safety glass with a layer of tree resin in 1905, the real credit for the discovery belongs to France in 1903. Edouard Benedictus (painter, composer, and chemist; 1878-1930) accidentally knocked over a glass flask while working in his lab, and was surprised to discover that it failed to shatter. A brief discussion with his assistant revealed that the flask had contained a small amount of liquid plastic (celluloid), which had evaporated, leaving a transparent layer of plastic on the inside of the flask. Because it was invisible, the assistant assumed the flask was clean and put it away without washing it. When the flask hit the floor, the layer of plastic held the shards together, preventing it from shattering. After numerous experiments, he received a patent for his invention in 1909.

As automobiles were relatively new in the early 20th century, Benedictus saw an application for this discovery in windshields. Many injuries from car accidents at the time were compounded by flying glass from shattered windshields, and his safety glass could save lives. Unfortunately, the developing automobile industry had no incentive to spend the extra money on his invention. If one car was safer but more expensive, the average consumer would probably buy the less safe but cheaper model. It wasn't until 1937 that safety glass became mandatory for car windshields, and few manufacturers included the option before then. It is perhaps interesting to note that windshield wipers were also made mandatory in this year.

Benedictus also developed the idea of using a gelatin layer to adhere the plastic layer to the glass layer in 1910, creating the first three layer safety glass, called Triplex. This version of safety glass was the first to see real widespread use – during World War I. The military quickly saw the advantages of using glass that would not shatter in such delicate and high-risk applications as airplane windshields and gas masks.

Early types of safety glass were made with celluloid, but celluloid becomes brittle and discolored with age. Modern safety glass uses vinyl, specifically polyvinyl butyral, or PVB. Today, the Dupont corporation holds the Benedictus awards in honor of his discovery, which recognizes "the innovative and outstanding use of laminated glass in architectural projects worldwide".

Some clarifications: Sources vary on some minor but significant points. As far as I can tell, Edouard Benedictus first discovered the laminating process for safety glass in 1903, but John Crew Wood beat him to the patent office with his tree resin version in 1905. Benedictus patented his first celluloid version in 1909, and then added a gelatin layer to patent Triplex in 1910. No manufacturers seemed interested in using safety glass in windshields before 1920, although it is unclear who was the first to do so before it became mandatory in 1937.

http://ourworld.compuserve.com/homepages/traffic_safety/history1.htm
http://www.exn.ca/Stories/2004/04/19/51.asp?t=dp
http://www.wfu.edu/users/pepier1/project.htm
http://applications.edmunds.com/ownership/safety/articles/43816/article.html