Picture the least interesting object you pass on a highway.
Two steel legs. A rectangle on top. A poster for a personal injury lawyer with a very confident expression.
Nothing about it looks valuable. It has no employees. It does not open. Nobody goes inside it. It sits in a weedy patch of dirt between a frontage road and a drainage ditch.
That object can generate more annual income than a decent house, on a footprint smaller than a parking space, with essentially no ongoing labor.
And here is the part that makes it genuinely fascinating.
In most of the country, you are not allowed to build a new one.
Not because it is expensive. Not because it is difficult. Because it is illegal, under federal and local rules that have been quietly tightening for sixty years.
Gif by adweek on Giphy
Which means every billboard standing today is part of a fixed and slowly shrinking supply of things that cannot be replaced.