As said below ... DO make sure they're microchipped before being let outdoors!
Most cats learn very quickly, my little Sabby learned how to use a catflap at 5 months old (yes, he was neutered by then), straight away! You could almost see the grin on his face! I 'taught' him by waving a treat at him from inside, to coax him through, and showed him how it swung open. That was it, really.
It's true about quieter roads though, which is kinda counter-intuitive. My bro has lost 2 cats to RTAs in the last 2 years, and he lives in a remote hamlet miles from anywhere! Trouble is, they (the cats, not my brother's family!) quite happily sit in the lane, then BAM! an unexpected car comes round the corner and it's goodbye kitty
I live close to a very busy road, but because there's a constant stream of fast traffic and noise, my lot won't go near it, and have learned to avoid car noises. It all depends on the cat really. My best friend's cat is car-shy, and has never lived anywhere other than a quiet cul-de-sac.