Dunno about hi-viz?
If you can't see a horse then you're just not looking 
I can recall at least one occasion locally, where I've come around a bend to be confronted with a girl on a horse coming the other way. Large dark brown horse with young lady wearing dark top and olive green body warmer, problem being behind them was a tall hedgerow (taller than horse and rider) - dark green foliage and dark brown trunks(?). It was quite a bright day too, on the last occasion.
I generally don't have a problem with horses being ridden on the road, except for piles of poop on corner entries, and the fact that I usually find them riding two side by side as I come around a nice corner! But some of the (horse) riders could do with a quiet word of advice.
I usually try to knock the bike up a couple of gears and bring the revs down, pass slow and wide and not rev the engine suddenly, but it can be a bit difficult when you spot them mid corner whilst lent over.
I find if you try to be courteous, they'll usually acknowledge with a wave and a smile.
Safe riding.
NC30Gnome

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