Just after you pass through Richmond, VA, northbound on interstate 95, you'll see a sign declaring that Washington is 100 miles ahead of you. If you're headed to Chevy Chase, maybe a few miles more, or Bolling AFB, a few miles less. But what point is the state of Virginia measuring from? That point would be rather specific; the monumental, and little known Zero Milestone.
Zero Milestone is a precise point in downtown Washington designated by a stone marker. Regarding road distance from Washington DC, all locations in the U.S. are measured from this specific stone maker.
Zero Milestone sits just due south of the White House. This stone was dedicated in 1923 after a temporary marker was put in place in 1919. It was the spot from which an Army convoy departed on its first transcontinental convoy of U.S. military vehicles over the Lincoln Highway. The route of Lincoln Highway was one of the first to traverse America coast to coast (now, much of present day US Route 30).
In this sense, the Zero Milestone was a recognition of the growing influence of the car on American life and culture. Its dedication came just years after the passage of the Federal Aid Road Act of 1916, the first federal highway funding bill to pass through Congress. And to a point, all road distances to Washington DC are measured from this singular mile marker. The original, Zero Milestone. By the way, the Army convoy did complete the 3,200 mile trip, in one day short of two months -- July 7 to September 6, 1919. The most famous of its participants; future President Dwight Eisenhower.