Just to do the devil's advocate thing: I live in the US, on a quiet,
residential one-way street in an urban area.

I dunno. I read your post and tried to imagine what it's like there...the main
hang-up for me is that you described the street as "busy." And that the cats-
"rarely cross it. It's a crap shoot, IMO, and the odds aren't in your favor
enough that I'd feel comfortable letting them out to roam. It's just that *one
time* you look out, and your cat is dead in the road. It only takes one
speeding idiot. You have to judge whether you're willing to take that risk. I
don't think I would. Secondly, are the other neighborhood cats. FIP is pretty
common here, there's really no vaccine for it. I wouldn't want mine hanging
around with other people's cats.
Just my .02

Sherry