What is the difference between safety glass & normal glass?

As early as the 1930's, safety glass was being used and tested for a variety of purposes including motor vehicles.

Once vehicles (like the model T) started getting upwards of 40 miles per hour (64 kmh), an important safety aspect became necessary. A windscreen, (that funnily enough was required to keep the wind of the drivers face), but also the debris like rocks being flung up, required a protective barrier that didn't break easily or often.

Cue the introduction of safety glass. This old video from Ford in the 1930's shows impact testing being done in a method that is still used today.