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.