Once upon a time, messages on highway billboards could be changed only by workers who climbed up to each board and painted or wall-papered on new messages. Today, increasingly, billboards are automated. The messages these billboards portray are displayed by a series of lights and can be programmed in from a computer.
When I read this story  in Thursday’s New York Times about automated billboards, I recalled the very first time I saw such a thing. It was in 1994 as I drove on a crowded highway in Dallas. The sighting of this billboard proved very helpful to me because I was then on my way to deliver a lecture on economics. When the subject of the minimum-wage arose during Q&A, someone asserted that most jobs "must be done," so most employers will have no choice but to raise their low-wage workers’ pay up to the legislated minimum.
The billboard then popped into my mind. Billboard-owners’ ability to substitute capital for labor means that billboard workers — the workers who once climbed up to billboards and changed the messages — are in fact no longer necessary.