It used to be just construction workers that wore high-visibility tops. Now they adorn police on traffic duty, tram/train customer service people, cyclists, truck drivers, builders, couriers, and so on.
They do make a person more visible (spectacularly so at night), even if they do little else. Railway industry people wryly observe that some of the more foolish of their number think that a high-visibility jacket will magically prevent them from being hit by trains.
It might be a bit of an arms race like visibility from four-wheel drives. How visible will they be if everyone wears them? Not at all, in crowds, I suspect.