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.
Speed limits in the United States by jurisdiction
Tennessee generally posts its rural interstates at 70 mph. These include all of the state's two-digit Interstates except Interstate 55 and Interstate 26. The only auxiliary interstates in Tennessee with a 70 mph speed limit are Interstate 155 and Interstate 840. 70 mph speed limits are allowed on all other controlled access highways, and are found on controlled-access portions of U.S. 51 and S.R. 22, 111, 386, and 396. In 70 mph zones, the default minimum speed limit is 55. Urban interstates are generally posted at 55 to 65 mph, however, portions of Interstates 24, 40, and 65 and S.R. 386 in Davidson County, as well as I-24 in Clarksville are posted at 70 mph. Four-lane divided highways are posted as high as 65 mph but are sometimes 55 mph. These are often hotbeds for speed camera enforcement leading into a municipality. 55 mph speed limits are allowed on two-lane state maintained roads, but some contain 45 and 50 mph limits, even in rural areas, primarily as a result of local policies. In East Tennessee, most mountain roads leading to North Carolina are poorly posted at 45 mph. Furthermore, two-lane roads such as US 321 and S.R. 91 have the same mountain and road width characteristics when leaving Tennessee into North Carolina or Virginia at 55 mph.