Mile markers begin at the most southerly or westerly point of a state. As a traveler moves north or east, the mile markers increase with every mile traveled.
Mile markers start with milepost zero. Mile markers can be used to help report road problems to local authorities. Interstate exits use mile markers for exit numbers. Exit numbers are determined by where the southern-most or western-most exit starts, and are the same number for both directions.
California is the only state that does not use mile markers. Instead, it uses a system that begins and ends at county lines, and in 2002 began erecting exit marker signs based on the mile marker system.