For the past several days, I've been working with my three-year old Percheron and getting him used to being around moving vehicles.
The horses are pastured along a moderately busy road, so they're pretty used to seeing cars and trucks go by. But I like to do some concentrated work on getting a horse used to being in close proximity to vehicles before I take my life in my hands, climb aboard, and ride on the side of the road.
The best way I know of to get a horse used to traffic is to play "chase" with a vehicle. I've been having my friend and neighbor, Bridgette, come over on her 4-wheeler and drive slowly around the pasture while Theo and I follow. As long as the vehicle in question is moving away from the horse, he generally makes a fairly quick transition from "Aaaaah! What is THAT!?" to "Hurry up, it's getting away."
We practice following until I can sit on the tailgate and lead him behind the vehicle. We also practice lungeing around the vehicle from all angles, with it sitting still and with it moving.
Sometimes I stand on it and practice bringing the horse close enough to be rubbed on, and then back him away from me to start all over again.
Once the horse is comfortable following the vehicle and working around it while it's sitting still, then we practice walking calmly while the vehicle makes runs up and down beside us -- getting him used to seeing things come up from behind him and come straight at him. I also like to practice having him stand or work quietly while the vehicle runs circles around us.
It generally only takes a few sessions before the horse could care less about most vehicles. But in my experience, traffic training is an important part of every horse's schooling.