Deadly serious. We have a few horses, and have ridden them out on the roads many times.
Some horses are more nervous of passing traffic than others, some riders are more nervousof passing traffic than others. My sister was on a horse 30 years ago whuch was struck by a heating oil lorry. It nearly cut the horse in half and died before a vet got there. My sister got away with her life, but was badly affected by it. The truck dtiver didn't start braking until after he had hit the horse