Why a horse gets scared on a roadside fence?
Why does a horse have difficulties in passing this type of fences or gates?
A horse has this scanning like type of view. His brains save pictures of the surroundings from last time he has passed a spot and automatically compare them to new ones. So he is receptible to environment changes that could mean a threat for him. Because he can't see much in depth, the vertical laths make a moving like effect, a visual illusion, that's confusing for the horse. It seems to him that there is something moving behind the fence or gate (a predator). Thus he gets scared more often compared to a wall like fence.
Give the horse time to smell the fence, to look over it and see that there's nothing threathening him. Don't push him to pass the fence, wait for him, while gently applying a little leg and seat aid for forward movement, repeatedly without increasing the intensity of the aid.
Praise every forward step or half of a step he makes. When you pass the fence, after some meters away from the fence if, he's calm, reward him, turn and repeat it once or twice. Only then proceed with your path.
Talk to him with a calm voice, breathe deeply all the time, because in that moment you're his herd and he will be mirroring you. Deep, slow breathing has a calming efect for you and your horse.