I am in the process of training my 4 year old to be road safe as well.
I have been hand walking him. I started around a relatively quiet neighborhood off a busy road. The first time he was a bit nervous. I didn't push it. I went as far as I thought he could handle, let him eat some grass, then turned around and went home. Then I did it again and went a bit further. Last time we went around the entire 1/4 mile block and then down a major road for about another 1/4 mile. No drama. He followed me along, even when the cars whizzed by. Hope this helps.