Horses are classified as a slow-moving vehicle in the California Drivers' Handbook:
Animal-Drawn Vehicles: Horse-drawn vehicles and riders of horses or other animals are entitled to share the road with motor vehicles. It is a traffic offense to scare horses or stampede livestock. Slow down or stop, if necessary, or when requested to do so by the riders or herders.The common-sense course of slowing down and accepting one's circumstance, despite the temptation to speed around the horse, turned out to be written into law.
After he had gone two blocks, the policeman stopped and waved the traffic around. Both he and the drivers behaved sensibly, a sign of hope for civic life.