In 1928 the New York State Education Department devised an initiative to mark places of historical significance, and over the next four decades, almost 2800 such markers were placed all over the state.
The signs themselves are marvels of design, in my opinion. Most of them feature dark blue backgrounds with gold raised block lettering and trim, though there are variations in color, lettering, and very occasionally shape, just to change it up, I imagine. The state discontinued the series in 1966 after high-speed travel on expressways became the norm.
This flickr page that assembles photos of the markers taken by various photographers illustrates the basic, simple and readable design of these signs.