South Dakota's state highways are assigned in a numbering pattern that followed that of the U.S. Highways followed upon their inception. EastâÂÂwest highways carried even numbers and increased from North to South â while northâÂÂsouth highways carried odd numbers and increased from east to west. This holds true only for two-digit highways. Three-digit highways follow the oddâÂÂeven routing, but they do not sequentially remain near a "parent" route as a spur or alternate route and instead are independent of any parent two-digit route.