The city is described in a mid-20th-century civics textbook as "a city of islands—islands created both by water and by abrupt valleys that can be traversed only by bridges." Already by 1948, 221,500 vehicles a day crossed the city's bridges across the Lake Washington Ship Canal and Duwamish River; except for the high Aurora Bridge (officially George Washington Memorial Bridge) across the Ship Canal, these were all drawbridges. This was before the construction of the Interstate Highways or State Route 520; the original Lake Washington Floating Bridge (opened 1940) provided the only road out of town to the east; construction of the Alaskan Way Viaduct, the first limited-access highway through the city center, was still underway.