The Zoo Freeway is a major north-south facility in western Milwaukee County, running 13.5 miles from the Hale Interchange in Greenfield northerly to the North Interchange in northwest Milwaukee. At the Hale Interchange, the I-894/I-41/US-41/US-45/Zoo Freeway meets the I-43/Rock Freeway and the I-894/I-41/US-41/I-43/Airport Freeway, while at the North Interchange, the I-41/US-41/US-45/Zoo Freeway intersects the STH-145/Fond du Lac Freeway. The Zoo itself carries the I-41, US-41, and US-45 route designations in its entirety, while the southernmost four miles are also part of the I-894 bypass route of Milwaukee.
The earliest Milwaukee County Expressway Commission plans included a freeway in at least part of the modern day Zoo corridor. The earliest proposals called for the "West Freeway" to serve more as a feeder route into the East-West Freeway, beginning at STH-15/National Ave and continuing northerly to the STH-100/Mayfair Rd interchange. The Zoo was envisioned as a relief route for STH-100 traffic. After the Milwaukee County Zoo was relocated from Washington Park in the 1950s—due, in part, to the planned freeway to traverse the portion of Washington Park where the original zoo was located—the West Freeway was rechristened to the Zoo Freeway.
Construction on the US-45 freeway began in the early 1960s at both the north end and the middle of the proposed freeway. In the middle of the corridor, the originally-planned 4.6-mile stretch was completed in 1963 between Beloit Rd and North Ave, with the southernmost three miles co-signed as I-894. On the north end, the portion of US-41/US-45/STH-100 along N 115th St in northwest Milwaukee from just south of Appleton Ave northerly to the new interchange with the STH-145/Fond du Lac Freeway was converted to expressway/freeway standards by 1965 with interchanges at Appleton Ave, CTH-PP/Good Hope Rd and the Fond du Lac Freeway. South of Appleton Ave, the highway narrowed back down to the original N 115th St (now Lovers Ln) alignment.
In 1966, the Zoo Freeway from Beloit Rd southerly to the Hale Interchange was opened to traffic as part of the completion of the I-894 bypass that year. A year later, the entire freeway was finished with the opening of the segment from North Ave northerly to the STH-145/Fond du Lac Freeway in northwest Milwaukee. The Zoo Freeway was now completed as we know it today.
In 2015, major highway route designations throughout Greater Milwaukee resulted from the debut of I-41 between the Illinois state line and Green Bay. To accommodate the unique pairing of I-41 with US-41, the route of the latter was adjusted to bypass the city along I-894 and US-45 using the Airport Freeway and Zoo Freeway. The US-41 designation was simply removed from the I-94/I-43/North-South Freeway between the Mitchell and Marquette Interchanges and from the I-94/East-West Freeway from the Marquette westerly to the Stadium Interchange. The route of US-41 along the Stadium North Freeway and surface streets through northwest Milwaukee was redesignated as an extension of STH-175. Formerly just designated as I-894/US-45 between the Hale and Zoo Interchanges, the southern portion of the Zoo Freeway is now I-894/I-41/US-41/US-45 while the northern part, previously just US-45 from the Zoo Interchange northerly to Appleton Ave as and US-41/US-45 from there to the North Interchange, is now signed as I-41/US-41/US-45.