SH-128 begins at the eastern terminus of SR 128 at the Washington–Idaho state line that separates the cities of Clarkston and Lewiston.

Every year, the Idaho Transportation Department (ITD) conducts a series of surveys on its highways in the state to measure traffic volume.

In 2011, ITD calculated that 4,702 vehicles per day used SH-128 between the Washington state line and US-12.

SH-128 has a short, 0.130-mile-long (0.209 km) spur route near its eastern terminus at US 12 in Lewiston.

[1] The spur route travels southeast from SH-128 to US 12 westbound and serves as an extension of the intersection of the two highways to the north.