Unmarked

An unmarked term, in linguistics, is one which is used is a general sense. For example, "man hour" refers to the amount of work one person can do in one hour. It does not specify a male human being, despite protestations by feminist language reformers. The term "person hour" has accordingly not gained currency.

A road sign saying "men at work" does indeed employ the connotation of big, burly, heavily-muscled male human beings. That is the sort of worker typically employed in heavy construction. The point of the sign is to prompt the necessary caution by drivers, alerting them to the presence of construction workers along or on the road. A phrase like "people at work" would fail at this purpose, because it connotes shopkeepers and secretaries. You might as well say "downtown area".