The Zero Milestone is a zero mile marker monument in Washington, D.C. intended as the initial milestone from which all road distances in the United States should be reckoned when it was built. At present, only roads in the Washington, D.C. area have distances measured from it.
From the inscriptions:
STARTING POINT OF SECOND TRANSCONTINENTAL MOTOR CONVOY OVER THE BANKHEAD HIGHWAY, June 14, 1920
STARTING POINT OF FIRST TRANSCONTINENTAL MOTOR CONVOY OVER THE LINCOLN HIGHWAY, July 7, 1919
DEDICATED June 4, 1923
Links: virtualglobetrotting.com, en.wikipedia.org