Never too early, if you ask me. We always talk it out when crossing the roadway. "STOP. Look AAALLLLL ways. Is anyone coming?" and so on.
I was out with the kids the other day, and was glad that I had my wits about me. My p/t boy just darted out onto the road!! He only made it a step or two out, but it really reminded me how these things happen. If I hadn't been paying attention, and he had gotten farther, and a vehicle had been coming?? Yeesh.
What shocked me was that he is two and a half. Shouldn't he know better?? He does now... my scooping him up and using a very firm voice startled the bejeebers out of him!