Wisconsin Highway 78

State Trunk Highway 78 (often called Highway 78, STH-78 or WIS 78) is a state highway in the U.S. state of Wisconsin. It runs north–south in south central Wisconsin from the Illinois border near Gratiot to Portage.

Route description
WIS 78 begins at the Illinois state line, continued from Illinois Route 78 (IL 78). It enters Gratiot, where it merges with WIS 11 and leaves Gratiot. WIS 78 turns north, and runs concurrently with WIS 81 in Argyle before turning to go north in the Driftless Area. WIS 78 passes through Blanchardville. The highway runs concurrently with WIS 39 for less than a quarter of a mile (0.4 km) before the two highways part ways, and WIS 78 passes through Daleyville and enters Mount Horeb, crosses US 18/US 151, and meets WIS 92 at its western terminus.

WIS 78 continues north out of Mount Horeb through the Driftless Area. It enters Black Earth and merges with US 14 west and the routes run concurrently, leaving Black Earth. WIS 78 turns north. WIS 19 begins at this intersection and the two routes run concurrently for a mile, before WIS 19 turns to go east. WIS 78 runs parallel to the Wisconsin and Southern Railroad for several miles.

The highway merges with US 12 and crosses the Wisconsin River, entering Sauk City before it turns to go north, now running concurrently with WIS 60. WIS 60 turns to go east and WIS 78 continues out of Sauk City, paralleling the Wisconsin River for several miles. It merges with WIS 113, creating a wrong-way concurrency with WIS 113 south and WIS 78 north, and vice-versa. In Merrimac, WIS 113 turns to cross Lake Wisconsin with the Merrimac Ferry. The route continues for 12 mi before ending at an interchange with Interstate 39 (I-39) and I-90/I-94. At the interchange, the roadway continues as I-39.

History
In the 1920s, WIS 78 was originally located on the Door County Peninsula on what is now WIS 57.

In 1963, the segment of WIS 78 from the new I-90/I-94 freeway interchange south of Portage northerly to WIS 33 was improved to a four-lane divided facility. A year later, this four-lane divided highway (and the WIS 78 designation with it) was extended northerly across the Wisconsin River then bypassing Portage to the west before merging into the existing two-lane route of US 51 north of the city. The entire four-lane divided portion of WIS 78 from I-90/I-94 northerly to US 51 was shown as a full freeway with interchange beginning with the 1966 official state highway map, so it can be assumed the conversion to freeway occurred about this time (c.1965). In more recent decades, the short segment of WIS 78 freeway was the only portion of the route from I-90/I-94 at Portage northerly past Wausau that was not signed as part of US 51, although signs along the route did indicate "To US 51". Then in 1996, US 51 from Rothschild (south of Wausau) southerly to Portage was concurrently designated as I-39 and to connect this new Interstate route with nearby I-90/I-94, the entire WIS 78 freeway from US 51 north of Portage to I-90/I-94 was redesignated as part of I-39, with WIS -78 losing approximately 8 mi of length in the process.