Milestones interest me. I suppose I first encountered them in that odd card game Mille Bornes:
Supposedly this was based on fact, with a Golden Milestone, the Milliarium Aureum in the middle of the Roman Forum acting as the starting point from which all Roman roads were measured.
Actually the Golden Milestone was probably just gilded bronze, and most Roman milestones do not mention distance from Rome, but it was still an interesting idea. It was later copied by the Byzantines with a marker called the Milion at Constantinople, and dubiously by a stone artifact called The London Stone, in central London.
So it stands to reason that eventually somebody in Washington D.C. would figure we needed an official central milestone there. I am just a little surprised it took until 1919 for it to happen.