Downtown Boston's streets grew organically, so they do not form a planned grid, unlike those in later-developed Back Bay, East Boston, the South End, and South Boston. Boston is the eastern terminus of I-90, which in Massachusetts runs along the Massachusetts Turnpike. The elevated portion of the Central Artery, which carried most of the through traffic in downtown Boston, was replaced with the O'Neill Tunnel during the Big Dig, substantially completed in early 2006.