The Leonard P. Zakim Bunker Hill Memorial Bridge (or Zakim Bridge) is a cable-stayed bridge across the Charles River in Boston, Massachusetts. It is a replacement for the Charlestown High Bridge, an older truss bridge constructed in the 1950s. Of ten lanes, the main portion of the Zakim Bridge carries four lanes each way (northbound and southbound) of the Interstate 93 and U.S. Route 1 concurrency between the Thomas P. "Tip" O'Neill Jr. Tunnel and the elevated highway to the north. Two additional lanes are cantilevered outside the cables, which carry northbound traffic from the Sumner Tunnel and North End on-ramp. These lanes merge with the main highway north of the bridge.