For the trucking industry the system was a boon. Trucks had been siphoning business from the railroads for decades, and the interstates contributed to a further withering of the nation's rail network by enabling trucks to travel hundreds of miles overnight to make a delivery. By the end of the 20th century, more and more American manufacturers had adopted a Japanese production system that dispenses with big stockpiles of materials. Instead, parts and supplies are delivered to a factory—generally by truck—at the exact moment when they are needed. This so-called just-in-time approach, which yields big savings in inventory expenses, turned the nation's highways into a kind of virtual warehouse. Sometimes trucking firms partner with railroads by piggybacking trailers on flatcars for long-distance legs of their journeys, but America's highways have the upper hand in freight hauling, as they do in the movement of people-far more so than in most other developed countries. Today, about 70 percent of all freight deliveries in the United States are made by trucks.

Highways continue to engender more highways by their very success. As traffic grows, engineers are working to improve pavements, markings, crash barriers, and other design elements, and they wage an unending war against congestion, sometimes by tactics as simple as adding lanes or straightening curves, sometimes with megaprojects such as the digging of a 3.5-mile, eight-lane tunnel beneath downtown Boston. It's a journey with no end in sight; Americans crave mobility, and wheels will always need roads.