The organized system of Wisconsin State Trunk Highways (typically abbreviated as STH or WIS), the state highway system for the U.S. state of Wisconsin, was created in 1917. The legislation made Wisconsin the first state to have a standard numbering system for its highways. It was designed to connect every county seat and city with over 5000 residents. The original system numbered the roads starting at 11 and ran up to 199. The 1917 law required the creation of a distinctive state route marker, which was initially an inverted triangle. "...it soon became apparent that (the triangle) served no really useful purpose after the public had become educated to the idea of the numbering. The long vertical axis made it necessary to use a lot of space, much of which was useless."[1] Distinctive route markers were eventually redesigned to use a rounded rectangle for better space usage, but the inverted triangle remains superimposed, as a vestige.[1]
These routes were altered significantly in 1926 when United States Numbered Highways were commissioned. It was altered to include communities with at least 2,500 residents.[2] Further alteration occurred when the Interstate Highway System was commissioned in the state.[3]
North of Milwaukee transferred to US 141 (short section in Milwaukee) and US 41 in 1926; replaced by WIS 42 (this section was swapped with WIS 32 in 1951)
West of Madison replaced by US 18 in 1926; truncated to current ends in 1947 (section east of Watertown already part of US 16 (now WIS 16) and section west of Mazomanie already part with US 151)
32nd Division Memorial Highway;[4] swapped with WIS 42 south of Howards Grove in 1951; until 1989, followed CTH-PP, CTH-W, and CTH-67 from De Pere to Kiel
One section transferred to WIS 165 (now supplanted by realigned US 41/US 45) in 1926, sections concurrent with US 45 and US 151 in 1934; concurrency with US 45 eliminated in 1953, when the section south of the concurrency was renumbered WIS 145; concurrency with US 151 eliminated in 1965.
Renumbered WIS 14 because of the creation of US 61, but split into two and renumbered as WIS 81 and WIS 15 (now I-43) because of the extension of US 14 into Wisconsin
Replaced by extension of CTH-F and local roads; formerly ended at WIS 83 in North Lake; this section became CTH-VV in 1984, and the highway was extended south to WIS 190 in 1999.
In 1923, section from Ellsworth to Martell became part of WIS 46 (now US 63), Martell to Spring Valley became part of WIS 116 (now WIS 29), and Spring Valley to Glenwood City was given to the county (but later became WIS 128) in exchange for rerouting southeast to Menomonie; rerouted to bypass Glenwood City in 1947
Wisconsin Highway 318 was planned to be extended south totaling 6 miles in length, forming a complete bypass and ending at Wisconsin Highway 59. However, U.S. Highway 18 used the whole bypass instead.