FWIW this is probably what we would do if our yard was (or could be) fenced in completely. Cars are like 75% of my concern about letting them out and other animals are like 20%, so eliminating the first and almost eliminating the second (hawks are still a potential problem, I guess) would make me feel a lot more at ease about it. As it is we've tried taking them out on leashes but they either sit there like lumps because they don't like it or slip the leash and make us chase them around for a while. One of our cats makes a break for it if we aren't careful, and when he gets out I pretty much just panic and we bring him back in ASAP. I don't doubt that he'd come back probably by the next mealtime (if it were ten days I'd be a wreck, jesus), but who knows what might happen before then. Also our cats are in their adolescence, the golden years of bad decisions in any species, which sure doesn't make me more confident of his chances in the wild.