In general horses can handle both noise and movement well, but they don't like sudden noise or movement, and are very wary of anything unusual. If I am approaching horses on a bike I slow down in advance, make sure I'm visible, moving steadily and making some sort of regular sound. If my bike is quiet or the wind is against me I'll talk to them and keep talking as I move past. Horses like predictable Some horses will actually spook more at "discovering" something strange waiting quietly alongside the road, than something moving.