I lost several cats on the road years ago and although I wanted to keep the remaining cats safe I didnt want to keep them indoors, it would have been unfair to them - so I had a large cat run built. I've since moved and now they have a safely fenced area of the garden, but also have access to the rest of the garden when I'm around.
Persoanlly, I think its good for cats to have some outdoor space, but with your busy road it doesnt sound safe to allow any new cats to roam freely - so, as others have mentioned, perhaps you could consider a cat run, attached to the house so the cat/s can come and go as they please, or, you might be able to use any existing fencing, together with safety fencing to provide a safe enclosed garden.