State Route 123 begins at Sunnyside Junction on US-6/US-191, about 15 miles (24 km) southeast of Price.

It turns to the northeast and terminates shortly after it enters Whitmore Canyon.

[3] The road from Sunnyside Junction through Sunnyside to Columbia (now all part of East Carbon-Sunnyside) was added to the state highway system in 1931, initially numbered SR-124[4] but changed to SR-123 in 1933.

[5] The route was shortened in 1935 by moving the eastern terminus back into Sunnyside,[6] and splitting off the portion from Sunnyside to Columbia as SR-124.

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