Keep him on a leash! And when you walk him, follow the same route -- one that goes AWAY from the busy street.
I live on a busy four-lane roadway. Once, Ozzie snuck out through a cracked door (thought he was in a carrier, but he wasn't). Caught a glimpse of him as he did it and ran after him. Luckily, instead of heading for the street, he made an immediate left turn up the driveway and started heading for the back yard -- our usual walking route.
This suggestion is in addition to getting him to learn STAY.
