Most of the state highways comprising the 11,800-mile system are posted with either Interstate-, US- or State Trunk Highway-numbered designations. Even though each of these different types of route designations have different route markers, they are marked and under the jurisdiction of the Wisconsin Department of Transportation (WisDOT). Each of these three types of state trunk highways are defined below:
Wisconsin is home to five "mainline" Interstate routes, with original construction commencing in 1956 in Waukesha County. Originally, however, the state was to only have two: I-90 and I-94. In time, WisDOT was able to convince the federal government to okay the completion of the US-141 freeway between Milwaukee and Green Bay and designate it as I-43. In the 1980s, the I-43 designation was extended southwest of Milwaukee to Beloit. Wisconsin's fourth Interstate, I-39, debuted in the 1990s, first from Portage to Wausau and later extended southerly past Madison into Illinois at Beloit. The newest Wisconsin Interstate—a unique route amongst Interstate Highways and an oddity in more than one way—I-41 debuted in 2015. Wisconsin does not make use of the Interstate Business Connection (Business Loop, Business Spur).
It took the federal government almost ten years to form a national system of route-numbered highways after Wisconsin created theirs. Today, the state is home to fourteen US routes: 2, 8, 10, 12, 14, 18, 41, 45, 51, 53, 61, 63, 141 and 151. With the coming of the Interstates, only one US route has been decommissioned: US-16. Beside US-16, the only other notable loss of US-route mileage was the southern portion of US-141 between Green Bay and Milwaukee, which was replaced by I-43. A few historical US Highways were decommissioned earlier on, such as US-110 and US-118.
State Trunk Highways
Wisconsin was the first state to set up a statewide posted route numbering system in 1917. Originally, state routes in Wisconsin were numbered only from 10 through 75—single-digit numbers were not originally chosen in an attempt to prevent some cities from claiming they were optimally located on "Highway 1," for example. Since 1917, the state has chosen to refrain from using single-digit state route numbers. During various expansions of the state highway system, route numbers into the 190s were used. Routes greater then 199 are newer designations, mostly running along other relocated or decommissioned routes. One of the noticable idosyncracies is the meandering nature of some of Wisconsin's state highways. Just pick one of the state's longer routes and more than likely you will find many extended stretches concurrently designated with other highways and some meandering behavior.
County Trunk Highways
Each county in Wisconsin maintains its own Country Trunk Highway (CTH) system, where routes are designated by letters instead of numbers. While the designation of some county highways may cross into two or more counties, any individual letter will appear many times in different parts of the state. The County Trunk Highway designations can be one-, two-, or three-letters in length. This system has its beginnings in 1921 and by 1924, each county in the state laid out a system of county-maintained routes, exclusive of the state trunkline system—but without legislative authorization. Then in 1925, the Wisconsin state legislature authorized the system, which had been in place already in some counties for more than four years. Today, there are occurrences where some imagination has gone into the designation of county highways. For example, the County Trunk Highway which runs along the county line between Kenosha and Racine Counties is CTH-KR, the former US-12/US-18 in Madison running along Broadway is CTH-BW (for BroadWay), and CTH-LO (formerly STH-99 until January 1999) was named in honor of former Waukesha County Board Chairman Lloyd Owens.
Additional types of "other" highways in Wisconsin, such as the Rustic Road System, Great Lakes Circle Tours and others are detailed in the Other Highways section of this website.