Just a reminder: eveyryone should carry a hammer to break car windows

[Stinky The Clown]

47. That's a good tool because of the seat belt cutter, but maybe a better way is a center punch

A spring loaded center punch, to be exact. Harbor Freight has them for just a few bucks, but really you can get them anywhere tools are sold.

In practice, you need only press the point against the glass. No need to cock or arm it. Just push it into the glass. The internal spring sets itself with pressure and when it reaches full travel, releases with more than enough force to break the glass. The required strength to do this is remarkably low. Even the elderly or physically challenged, it seems to me, have a better chance for success that even with that pointed hammer.

Here is a youtube showing it in action.