The longest: The Pan-American Highway
The Pan-American Highway winds some 20,000 miles through 17 countries, from the tip of North America in a small town in Alaska to the tip of South America in a small town in Tierra del Fuego, Argentina. It even cuts through the Nazca Plateua, as shown here.
A 90-mile break (the Darién Gap) in Panama stops the highway from being nonstop. Otherwise, it's clear sailing through high mountains, deserts, rain forests and a lot of asphalt.