I think making the noise at the side of the road when the cat is in the road is a good idea, If I am getting in my car and my cats are comming towards the road, I would always rev my engine to scare them away from the road.

Have you tried playing in the garden with the cat? They normally like to be where you are, so if you are in the garden, they are more likely to stay there. Also, try not to let him out at dawn or dusk, keep it to the middle of the day, they are more lazy in the middle of the day, and more likely to do a bit of sunbathing than go for a wonder.