Meowy Catkin
Meow!
We have a lot of low flying helicopters here and they definitely give you more room if you are wearing large amounts of Hi Viz, so it's not just about being on the roads and meeting cars.
I'm going to put my tin hat on now..................I have major concerns about hi vis because it puts the responsibility on the horse and rider. Every driver, bar none, should be driving on the road they can see, and concentrating on what they can see there. Movement catches they eye more than stationary objects, so a moving horse should be easily spotted. And there are other non vehicular road users who may not have access to hi vis. Escaped animals, elderly pedestrians, children, if we train drivers to just look for users in high vis they begin to get a mindset that ignores anything that isn't dressed in it.
That is of course in broad daylight, in dusk and poor visibility that is another matter