But cars had the irresistible advantage of flexibility, allowing drivers to go wherever they wanted, whenever they wanted, provided suitable roads were available. Efforts to accommodate motorized travel were soon launched by all levels of government, with particular emphasis on relieving the isolation of farmers. Beginning in 1907 the federal Office of Public Roads built experimental roads to test concrete, tars, and other surfacing materials. The agency also trained engineers in the arts of road location, grading, and drainage, then sent them out to work with state highway departments, which selected the routes and set the construction standards. Federal-state partnerships became the American way of road building, with the states joining together to harmonize their needs. When the United States entered World War I in 1917, trucks carrying extra-heavy loads of munitions and other supplies pounded many sections of highway to ruin. Even so, shippers were so impressed by their performance that the trucking industry boomed after the war, and new highways were engineered accordingly. During the 1920s, states increased the recommended thickness of concrete pavement on main roads from 4 inches to at least 6 and set the minimum pavement width at 20 feet. Extensive research was done on soil types to ensure adequate underlying support. Engineers improved old roads by smoothing out right-angle turns and banking the curves. At the same time, much research was done on signs, pavement markings, and other methods of traffic control. The first four-way, three-color traffic light appeared in Detroit in 1920. Europe provided some compelling lessons in road construction. In Italy, whose heritage included raised paved roads that allowed Roman armies to move swiftly across the empire, private companies began to build toll highways called autostrade in the mid-1920s. Although not especially well suited for fast-moving traffic, their limited-access design minimized disruption of the flow, and safety was further enhanced by the elimination of intersections with other roads or with railways. These features were also incorporated into the first true expressways, the national network of autobahns built in Germany between 1929 and 1942. The 1,310-mile system consisted of twin 30-foot-wide roadways separated by a grassy central strip, which significantly boosted capacity while allowing higher speeds.
The United States adopted the four-lane, limited-access scheme for relatively modest highways in Connecticut and California in the late 1930s and then produced a true engineering masterpiece, the Pennsylvania Turnpike, whose initial 164-mile section opened in 1940. A model for future high-speed, heavy-duty routes, the turnpike had a 10-foot median strip and a 200-foot total right-of-way. Each lane was 12 feet wide; curves were long and banked; grades were limited to 3 feet in a hundred; feeder and exit lanes merged smoothly with the main traffic streams; and the concrete pavement was surpassingly sturdy—9 inches thick, with a reinforcement of welded steel fabric. Travel time between Philadelphia and Pittsburgh was reduced by as much as 6 hours, but not for free. The Pennsylvania Turnpike was a toll road, and it did such an active business that many other states soon created their own turnpike authorities to construct similar self-financing superhighways. The United States adopted the four-lane, limited-access scheme for relatively modest highways in Connecticut and California in the late 1930s and then produced a true engineering masterpiece, the Pennsylvania Turnpike, whose initial 164-mile section opened in 1940. A model for future high-speed, heavy-duty routes, the turnpike had a 10-foot median strip and a 200-foot total right-of-way. Each lane was 12 feet wide; curves were long and banked; grades were limited to 3 feet in a hundred; feeder and exit lanes merged smoothly with the main traffic streams; and the concrete pavement was surpassingly sturdy—9 inches thick, with a reinforcement of welded steel fabric. Travel time between Philadelphia and Pittsburgh was reduced by as much as 6 hours, but not for free. The Pennsylvania Turnpike was a toll road, and it did such an active business that many other states soon created their own turnpike authorities to construct similar self-financing superhighways. As the nation's highway network grew, the challenge of leaping over water barriers inspired some structural wonders. One was the George Washington Bridge, which opened in 1931. To connect the island of Manhattan with New Jersey, Swiss-born engineer Othmar Ammann suspended a 3,500-foot, eight-lane roadway—the longest span in the world at the time—between a pair of lattice-steel towers on either side of the Hudson River. Special machinery spun and compressed the 105,000 miles of wires that went into the cables, and everything was made strong enough to support a second deck added later. In 1937, San Francisco was joined to Marin County with an even longer suspension span—4,200 feet. The Golden Gate Bridge, designed by Joseph Strauss, was built to withstand the swift tides and high winds of the Golden Gate strait. One of its tower supports had to be built almost a quarter-mile from shore in water 100 feet deep. A million tons of concrete went into the massive anchors for the cable ends.
The United States would eventually need half a million highway bridges, most of them small and unmemorable, some ranking among the loveliest structures ever created. Roads, too, aspired to beauty at times. During the 1920s and 1930s, parkways that meandered through pristine landscapes were laid out around New York City, and the National Park Service constructed scenic highways such as Skyline Drive along Virginia's Blue Ridge Mountains. In general, however, highways have done far more to alter the look of America than to celebrate it.
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