1st thing is to never ever allow him to go out the door on his own again. Even for potty breaks. Make him sit and stay at the door for a minute or two, and then say OK! Give him a treat, and let him out. This is so that he knows that every time he sits and waits by the door before going out, he will get a treat.
If he's still bolting out the door, I think you have no choice but to confine him to a space that doesn't let him anywhere near the door. Dogs dying in car accidents is a huge problem these days--it is horrifying. I saw large dog at our vet's once... he was run down by a car. It was horrible. The dog was crying out in pain the entire time.. imagine how much worse it would be if it were a small dog?
Cadence the tricolour Papillon (DOB: July 2nd, 2009) & Lyra the blenheim Cavalier (DOB: April 28th, 2011)