This mark is exceptionally easy to reach. It is on the Ellipse in front of the White House in Washington DC. Searching any online mapping service for "Washington, DC" will result at this milestone. The marker is on the south side of eastbound E Street NW.
Engraved on the monument are the following: on the north side, "ZERO MILESTONE"; on the west side, "STARTING POINT OF FIRST TRANSCONTINENTAL MOTOR CONVOY OVER THE LINCOLN HIGHWAY, JULY 7, 1919"; on the east side, "STARTING POINT OF SECOND TRANSCONTINENTAL MOTOR CONVOY OVER THE BANKHEAD HIGHWAY, JUNE 14, 1920"; and on the south side, "POINT FOR THE MEASUREMENT OF DISTANCES FROM WASHINGTON ON HIGHWAYS OF THE UNITED STATES".
A bronze compass rose sits atop the monument, originally engraved with "THE U.S. COAST AND GEODETIC SURVEY DETERMINED THE LATITUDE LONGITUDE AND ELEVATION OF THE ZERO MILESTONE." but which has been worn away through weathering and tourists.
The history of the Milestone is long and storied - both Wikipedia and the Department of Transportation have pages on it. Briefly, however, the stone was conceived by Good Roads advocate Dr. S. M. Johnson as a spot "from which all road distances in the United States and throughout the Western hemisphere should be reckoned" in the tradition of Rome.
A temporary Milestone was place in 1919, and from this, two journeys were undertaken, one in 1919, the other in 1920, from Washington DC to California (the earlier went to San Francisco along the Lincoln Highway, while the latter went to San Diego along the Bankhead Highway).
These travels proved extremely difficult at places, and helped cement the importance of good roads among Federal officials. And while the Milestone was never extensively used for its intended purpose, it did serve as a starting point for Good Roads legislation in the United States.