I have found that riding with traffic is preferable to a horse that is nervous about vehicles. Traffic that is coming at the horse is more of a concern than traffic coming from behind. On coming traffic will be farther away, since it is on the other side of the road, and easier to deal with. If traffic from behind produces nervousness the horse will tend to move in the same direction as the vehicle. Riding facing on coming traffic, as a pedestrian, can produce unwanted action, in that in an effort to get some room to deal with the approaching monster, we get 180 degree spins, and sometimes rearing.
Picketing a horse on grass in the bar ditch where there is traffic will help desensitize it to traffic.
Some things that work for me.