I live in the outskirts of a very large city. I have all my cats chipped, and collared at all times even though they are indoors only. They wear their rabies tag license nos, and our personal contact information.
I also have a 3 door exit system (glass door, wood door, a foyer, and then our door) so it's difficult for them to get out, but it HAS happened once. The cat did not get far, but had she leapt at the right time, she would've been another missing kitty in the neighborhood.
As far as collars, we tried the safety ones with the plastic clips and they kept coming off every time they were playing so now I get buckled ones that release if enough weight is put on them. They are more secure for the everyday, but every now and again, they still manage to get them off when they are playing.