The best place in the country to appreciate the marvels of our interstate highway system is heading west out of Denver on I-70 in Colorado. The road climbs and dips at a steep grade, taking cars across the Rockies, a range that includes some of North America’s tallest mountains. Fifty or so miles out of town, the Eisenhower-Johnson Memorial Tunnel, which takes travelers underneath the continental divide, marks the highest point in the entire interstate system, at 11,155 feet. Building the first of the memorial tunnel’s two bores, the one named after Eisenhower, cost more than $100 million, an extraordinary sum at the time.