Complex sentence: The Zero Milestone is a 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, but now only roads in the Washington, D.C. area have distances measured from it.
Split into simpler sentences: The Zero Milestone is a 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.