The Wisconsin State Trunk Highway System is the state highway system of the U.S. state of Wisconsin, including Wisconsin's segments of the Interstate Highway System and the United States Numbered Highway System, in addition to its other state trunk highways. These separate types of highways are respectively designated with an I-, US, or STH- (or WIS) prefix. The system also includes minor roads designated as Scenic Byways, four routes intended to promote tourism to scenic and historic areas of the state; and as Rustic Roads, lightly-traveled and often unpaved local roads which the state has deemed worthy of preservation and protection. The state highway system, altogether totaling across all of Wisconsin's 72 counties, is maintained by the Wisconsin Department of Transportation (WisDOT). Highway systems Interstate and U.S. highways The state of Wisconsin is served by eight Interstate Highways, consisting of five primary routes and three auxiliary routes. The first part of this system was constructed in 1956, and its most recent expansion took place in 2015, with the addition of I-41 to the system. Wisconsin's longest Interstate Highway is I-94, at , and its shortest is I-535, which extends only into the state. There are also fourteen United States Numbered Highways in the state of Wisconsin, which were designated beginning in 1926 and ending in the mid-1930s. The routes of the U.S. Highway System in Wisconsin have remained essentially unchanged since U.S. Highway 16 became a state highway in 1978. There are also several business routes, usually maintained by local governments. State trunk highways The state of Wisconsin maintains 158 state trunk highways, ranging from two-lane rural roads to limited-access freeways. These highways are paid for by the state's Transportation Fund, which is considered unique among state highway funds because it is kept entirely separate from the general fund, therefore, revenues received from transportation services are required to be used on transportation. The majority of state highway funding comes from gas taxes and vehicle registration fees. Other state highways WisDOT also develops and maintains some minor routes, under the designations of Scenic Byways and Rustic Roads. Scenic Byways are minor roads, typically two-lanes, that travel through areas of "scenic and historic interest." Rustic Roads are lightly-traveled local roads, sometimes paved and sometimes dirt or gravel, whose rustic characteristics are intentionally maintained and preserved by WisDOT. Other highways in Wisconsin All 72 counties in Wisconsin maintain their own system of county