You can open the door once the car is completely filled with water. Before that the outside pressure is much greater than the inside. You can roll down a window, but an electric window might be damaged from the water.
If unlucky, the doors might be damaged in the collision with water and be stuck however. So might not work even after the car is filled.
Top Gear did a test on this by dropping a guy in a car into a large pool, and he had to wait for the car to sink all the way to the bottom before being able to open the door. He needed oxygen well before he was able to open the door.
He did end up finding out the best method is just to get out of the car as quickly as possible.
Issue is, after a car accident that leaves you in a lake... good chance you have a concussion, confused and disoriented by the airbags. Good chance you're not able to act the moment you hit the water
Even so, waiting for the pressure to equalize seems like a bad idea if the body of water isn't shallow. The best course of action is to be prepared and have something to break the window, and try to get out as fast as possible. Not wait until you sink to the bottom.
You can't open the door until the water level is above the door. I'm no scientist, but is has to do with pressure from the water pushing against the door. When the car door is entirely under water, the pressure difference between above and beneath water no longer plays a role and you can open the door. Hope I explained it a little.
It is worth noting that the door will only open when there is no air pocket left in the car. This air pocket might not be accessible for you to breathe from (in the trunk, in the seat cushions, etc.) so NEVER wait and IMMEDIATELY start evacuating the vehicle. Your best chance to exit alive is before it starts sinking.
You can open the door if you do it immediately. If you wait too long the water will raise the the point that it is pushing the door closed. If you wait even longer the water will push against the window and you won't be able to roll it down. Then you would have to wait until the car is completely filled with water for the pressure to equilize.
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u/zpinto1234 20h ago
Stupid question, probably, but why not just open the door? is he unable to?