if a car travelling on a bridgewith the windows up crashed through the barrier and fell into water, A.could you survive and B. for how long?

You would not be a ble to survive as water will still get into the cabin where you are sitting as the water will get into the engine and through the vents plus i don't believe car doors are not water tight when driven underwater

he woldn't be drivin under the water cos he fell in to the water not drove in it!!!

It doesn't matter mate, the water comes in through the engine, anyway. They dropped a car into a huge pool on Top Gear a while back, and found that:

A. The doors will not open while the water pressure outside is greater than that inside (ie while it is sinking)
B. Once the car has reached the bottom, pressure equalises, and the door can be opened.
C. The best thing to do if your car hits water is to open the door and get out as quickly as possible, because once the water's up to around halfway up the door, it's impossible to shove open.

The only car i've ever heard of that would help in that sort of situation is a WV beetle. The door seals (in good condition) are so good they are almost water tight. A friend of mine had a Beetle that he had restored and it would almost pop your ears if you shut the doors too quickly.
Given that the vents on a beetle are though pipework that emerge halfway up the interior passenger compartment, it has been theorised that a beetle should float for quite a while before sinking. The problem would be that as soon as you open the doors to get out, water would engulf the car and it would capsize