I-5 was originally created in 1956 as part of the Interstate Highway System, but it was predated by several auto trails and highways built in the early 20th century. The Pacific Highway auto trail was built in the 1910s and 1920s by the states of California, Oregon, and Washington, and was later incorporated into U.S. Route 99 (US 99) in 1926. I-5 largely follows the route of US 99, with the exception of a portion in the Central Valley of California. The freeway was built in segments between 1956 and 1978, including expressway sections of US 99 that were built earlier to bypass various towns along the route. US 99 was removed in 1972.