I've been involved in an accident. First of all, you're supposed to pull over to the right-hand edge or curb of the highway, clear of an intersection, and stop.
Some people get confused and stop in the lane of traffic, creating a very dangerous situation.
Others make left turns into private driveways or businesses, still others continue on their way, thinking the sirens aren't for them. None of those last few examples are of course the correct thing to do, and could lead to additional violations being charged. After you're stopped, the Officer will make contact with you, during which time you should be advised why you are bing stopped. That is your proof of ownership of the vehicle. It doesn't validate your registration, and if someone else gets hold of it, they could try and claim the car as theirs.