Strange though it may seem,
Horses are quite adept at coping with strange situations and generaly panic when the herd leader ( read stupid woman ) takes flight or demonstrates fear.
When riding out I always face the horse to the possible threat be it fast car , loud tractor or low rotorcraft by not demontrating fear the horse knows it ok and not a threat.
We get a lot of training flights ( low ) and normal traffic here so the nags are used to it , they also like to follow the tractor when topping fields. Of course no horse is 100% and owners should take responisbility in assesing risk factors. as for Hi viz.too much about ,would be pulling up for paper boys , road workers,plod,joggers,discarded vests on the road side and on and on