List of Interstate Highways
There are 70 primary Interstate Highways in the Interstate Highway System, a network of controlled-access freeways in the United States. They are assigned one- or two-digit route numbers, whereas their associated "auxiliary" Interstate Highways receive three-digit route numbers. Typically, odd-numbered Interstates run south-north, with lower numbers in the west and higher numbers in the east; even-numbered Interstates run west-east, with lower numbers in the south and higher numbers in the north. Highways whose route numbers are divisible by "5" usually represent major coast-to-coast or border-to-border routes (ex. I-10 travels from Santa Monica, California, to Jacksonville, Florida, traveling from the Pacific to Atlantic oceans). Additionally, auxiliary highways have their numbering system where a different number prefixes the number of its parent highway.
U.S. Route 1
With the construction of the Interstate Highway System in and after the 1950s, much of US 1 from Houlton to Miami was bypassed by Interstate 95. Between Houlton and Brunswick, Maine, I-95 took a shorter inland route, much of it paralleling US 2 on the alignment proposed for US 1 in 1925. Between Philadelphia and Baltimore, I-95 leaves US 1 to pass through Wilmington. Most notably, I-95 and US 1 follow different corridors between Petersburg, Virginia, and Jacksonville, Florida; while US 1 followed the Fall Line west of the coastal plain, I-95 takes a more direct route through the plain and its swamps. Although some of this part of US 1 was followed by other Interstates—I-85 between Petersburg and Henderson, North Carolina, and I-20 between Camden, South Carolina and Augusta, Georgia—the rest remains an independent route with four lanes in many places. By the late 1970s, most of I-95 had been completed, replacing US 1 as the main corridor of the east coast and relegating most of it to local road status.US 1 has six three-digit auxiliary routes. In numerical order, these are:US 101, despite its number, is not an auxiliary route, but rather considered a primary U.S. Route in its own right as major highway west of the former US 99 on the west coast of the United States.