Beware of golf balls

I was somewhat amused by a signpost I saw today as I walked along a river next to a golf course. It said "Beware of golf balls". I assume they do not mean standing on one.

A golf ball (which is about the same size as a golf ball) travels at up to 180mph after initial hit and around 72mph on its way down after reaching its peak height.

Just how on earth do they expect us to beware!