Well Bullit, the good news is according to Mythbusters you'd run out of air in a very short time, long before you had a chance to die painfully of starvation. You'd black out even faster if you were busy screaming.
I'm terrified of my car plunging off a bridge with the kids in the back seat. I've obsessed countless times over the best way to get them out of their carseats and out the car windows.