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.

Beginning in the 1920s, residential communities left the old urban trolley lines far behind and spread outward from cities via roads. Stores, factories, and other businesses followed, sometimes aggregating into mini-metropolises themselves. As roads were improved to serve commuters and local needs, the outward migration accelerated, producing more traffic, which required more roads—almost limitlessly, it often seemed. In the late 1940s, for example, California began building an extensive system of express highways in and around Los Angeles and San Francisco, only to have congestion steadily worsen and a major expansion of the freeway system become necessary just a decade later.