I watched the full Mythbusters show on this topic. Those who think you CAN do this, please send me your address now to send flowers.

Some things, joke about, I do not.

You have never tried to smash out, on dry land, a laminated windscreen - mostly now compulsory instead of the old 'toughened glass' types that would shatter with a stone on gravel roads. An 8 pound sledge hammer will bounce off, and there is a big chunk of plastic sandwiched between two layers of glass that will get in your way as you without air exhaust yourself trying to get out.

As for opening the side windows under water - if you have electric windows, forget it - water shorts out electrical devices...

As the Mythbusters found to their amazement before they did some simple back of the envelope calculations and found just how many tons of pressure are holding the door shut when the water was not even up to even the windows, write your will now.

The pressure on the windows when they were even partly submerged was sufficient to jam them immovable, burning out any winder motor, and made the hand winders useless.

The only possible way to exit the car was when fully submerged (provided that one window was not fully down when the car went into the water), was to wait till the trapped air fully equalized pressure with the car below the surface of the water before a door could be opened. They did it in a swimming pool with a scuba gear guy in the back seat and a spare breathing set for the driver in case things went wrong.

It was just possible before running out of breath (after a few practice runs), as long as you didn't panic and held your breath. Once the door was even partly open a crack, the trapped air bubble rose to the surface, and you still needed a lot of force to open the door enough to get out. A lot of people die like this each year around the world by being trapped.

Sufficient pressure to open a door could not be done even by bracing against the opposite door. Nothing would smash even toughened (forget the laminated stuff!) glass underwater, boots, torches, except the gadgets that impact a hardened steel point - there is a spring loaded one, and a manual hammer style.

I'm not making this up, folks...