In the southwest corner of Idaho, State Highway 55 starts at the junction with US-95, approximately west of Marsing. The highway heads eastward as it travels through Marsing, crosses the Snake River and the Snake River Valley AVA. The highway continues east to Nampa, where it meets Interstate 84 and US-30. The highway is cosigned with those routes as it heads eastward to Meridian. Highway 55 turns northward at Eagle Road and crosses the Boise River near Eagle. The junction with SH-44 is in Eagle, where the routes are briefly cosigned. East of Eagle, Highway 55 turns northward and climbs to the Spring Valley and over the Spring Valley Summit, at, then descends vertically on Horseshoe Bend Hill into Horseshoe Bend. The new multi-lane grade was completed in fall 1991; the old curvier road is to the west, long plagued by landslides and closures. North of Horseshoe Bend, the highway ascends the Payette River and passes through several recreational areas, which offer rafting, fishing, and camping activities. Highway 55 continues north, through the small communities of Banks and Smiths Ferry, then crosses the river via the Rainbow Bridge, originally known as the North Fork Bridge. SH-55 then climbs Round Valley Creek to Round Valley, and continues northward through the extended Long Valley of Valley County to the county seat of Cascade. It ascends a brief summit at at Little Donner, then descends to follow the east shore of Cascade Reservoir. The route continues northward to Donnelly, the turnoff for Tamarack Resort, located to the southwest, on the western shore of the reservoir. SH-55 continues north through the valley to McCall, at the south shore of Payette Lake, the host of many scenic, recreational, and winter activities. Meeting the south shore of Payette Lake, Highway 55 turns westward as Lake Street through McCall and its west "Lardo" area. West of town, the road climbs to the Red Ridge, where it reaches its maximum elevation of above sea level, immediately northeast of the Little Ski Hill, also known as Goose Creek Summit. further is the turnoff for the Brundage Mountainski area, four miles north on Goose Lake Road. Highway 55 then enters a narrow and twisty canyon, rapidly descending with Little Goose Creek to Meadows, then to its northern terminus, the junction with US-95 in New Meadows at in Meadows Valley. Highway 55, from Eagle north to US-95 in New Meadows, is the Payette River Scenic Byway, part of the National Scenic Byways Program.
History
The original State Highway 55 ran from US-2 in Colburn to what is now State Highway 200. This highway was decommissioned in October 1955, and is now Colburn-Culver Road. The current route was designated as State Highway 55 in September 1967, replacing former State Highway 72 and former State Highway 15. Idaho State Highway 72 has since been reassigned to another road. State Highway 55 was originally routed through downtown Boise, and followed the old alignment of State Highway 44 down State Street and 23rd Street to Fairview Avenue. From there it turned westward with US-20, US-26 and US-30. The highway continued westward on the Boise Connector then Interstate 180N, and continued west along Interstate 80N, where it followed its present alignment toward Nampa. Recently in late 2006, the route was realigned away from downtown Nampa when a new interchange, Interchange #33 along Interstate 84 was opened and provided a more direct connection to State Highway 55 from the Interstate. The segments of SH-55 following the former Nampa Boulevard and Caldwell Boulevard through Nampa have been decommissioned favoring the more direct routing. From there the highway followed its current alignment from that point to Marsing and its southern terminus at US-95.
