The Grand Army of the Republic Highway, U.S. 6, runs from Provincetown on the tip of Cape Cod to Long Beach, California.
Its history is interesting in the ways it was patched together. Bit of trivia: Route 6 was "the road" Jack Kerouac meant to take, but he got caught in a rainstorm on the Bear Mountain Bridge north of NYC, so made other plans.
Photo is the Sagamore Bridge built in 1935 over the Cape Cod Canal on US 6.