In my fairly limited experience of RTA's (sorry RTCs) the door is unable to be opened in even fairly minor collisions. Basically if you aren't able to get out of your car I'm going to be boarding and collaring you because of 'mechanism of injury'. If the doors are able to be opened then it is likely that the crash wasn't that bad and your car won't burst into flames. So if it is a crash where the doors are jammed - I'll be calling out the water-squirters in order to cut you out of your wreckage. In this case locked doors won't mean a thing. Personally, driving around Newham, where it is not unknown for people to be dragged from their cars, I'd keep the doors locked. In lonely woodlike places, feel free to leave your doors unlocked for ease of exiting - no self respecting mugger looks for victims in desolate places.