For a horsemen to travel on highways designed for automobiles, both horse and rider must trust one another. As motorists whiz by, Bill continues along at a crawl, watching for broken glass, holes, road kill, and anything else which may become a danger. When a problem becomes visible, Bill must decide which action to take.
If there is road kill up ahead, he may decide to stop and wait for an open spot in the traffic so that he can cross over to the median. If a vehicle with a flapping bag passes, Bill must take action and kick Blackie up into a trot, so the horse doesn't have time to panic.
Usually the most dangerous situation is the one you don't see coming. These are the situations where trust plays the biggest role. There's a decent amount of room in this scene, but on highways where the guardrail is closer to the pavement, there isn't any room for mistakes or second guesses.