How do you desensitize a horse to bicycles, vehicles, and moving objects on the trail?

Bicycles and vehicles are high-priority trail desensitization targets because they move unpredictably, approach from behind, make unusual sounds, and can appear suddenly at close range — a combination that triggers even well-desensitized horses that have not been specifically exposed to them. Clinton Anderson's approach to vehicle desensitization begins in a controlled environment before the horse ever encounters vehicles on a trail. He starts with stationary vehicles near the arena or paddock — parking a truck, bicycle, or ATV within sight but at a comfortable distance while working the horse. As the horse habituates to the stationary object, he has an assistant move the vehicle slowly at a distance, then progressively closer, then at varying speeds. For bicycles specifically, the approach combines visual desensitization — getting the horse comfortable with the appearance of a bicycle at a standstill — with motion desensitization. A bicycle moving slowly is less frightening than one moving fast, and one moving away from the horse is less frightening than one approaching from behind. Anderson sequences the exposure from least threatening — stationary bicycle in view — to most threatening — bicycle approaching from behind at normal riding speed. Warwick Schiller's contribution is the observation that horses which have not been exposed to bicycles before encountering them on a trail are not being unreasonable in their fear response — the bicycle genuinely is an unfamiliar stimulus that pattern-matches to a predator in speed and approach. His practical advice is that trail exposure should follow controlled exposure: desensitize to the bicycle in a familiar environment first, then encounter it on the trail where the horse is already somewhat activated, rather than relying on the horse to manage a first bicycle encounter in an already stimulating environment.