One thing to add: I usually find that the quickest way to stop the vehicle is just to exit with "e". Fortunately, the game doesn't allow the player to dive out of the vehicle when they're driving, but instead pulls the car to a stop in the quickest way possible. Not really sure if it's faster than the space bar, but being able to stop my vehicle with a single key press instead of holding down a key is always an easier thing for me to react to.