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.
Highway systems by country
Many paved highways for vehicles are part of the official National Highway System of the U.S.. Paved highways in the "U.S. Highway" system (for example, U.S. Highway 53) can vary from two lanes wide (one lane each direction), shoulderless, roads with no access control, to multi-lane high-speed controlled-access highway, such as the Interstate Highways. These roads are usually distinguished by being important, but not always the primary, routes that connect populated areas. (Sometimes, the primary route is a State Highway.) Since their inception many decades ago, the construction of "U.S. Highways", and their major improvements, have been paid for 50% with Federal funds, especially from motor fuel taxes, and 50% with State funds from whatever tax resources that the state has. Thus, the system of "U.S. Highways" has always been an equal partnership between the Federal Government and the State governments. This was a plan that changed dramatically with the advent of the Interstate Highway system beginning in the 1950s, but do not forget that the system of "U.S. Highways" continued to be upgraded under the 50%-50% funding. Highways continue to be widened, old bridges continue to be replaces with newer and better ones, and so forth.