For my son when he was two/three, street and parking lot rules were laid out with a big dose of fear/reality. I found a big SUV parked in a parking lot and had him stand at the bumpers. He could easily see how the driver can't see him. I also pointed out dented cars and we talked about the force/strength/power necessary to bend the metal. He quickly put it together that his 40lb. squishy body would not handle a collision well.
At four, he is very careful in parking lots and crossing the street.