Only let your cats out if your garden is all fenced. Even then, it is necessary to supervise. As you say you live close to a busy street, I would keep the cats indoors!!

My 2 eldest cats are allowed outdoors, but only in our garden. Inka cannot escape due to her handicap, and Maya is to scared to escape. I don't know yet what we will do with both other cats, we will give it a try this summer.