Funny enough, I almost turned mine on today while on the rail trail. Fairly busy under heavy overcast skies and long stretches on the trail where the trees created a tunnel. Quite dark in the shaded passages. But I did not, I did turn it on when I came out onto the street. 200 Lumin. Was almost home when a fella walking towards me on the sidewalk, I was on the road, shaded his eyes ! I guess on a cloudy day the light really shines out and it is by no means a powerful one by todays standards.

Getting back to the OP. I turn mine off when hitting the trails just to save battery, never gave much thought to it being a hazard. Can't imagine how a horse might react to a bright strobe out on the trails.