It is the primary highway to Sun Moon Lake, a popular tourist destination.

The road between Tataka and Namasia was not built due to environmental and ecological issues.

21 begins at Dongshi District, Taichung City, then passes through Guoxing, Puli, Yuchih, Sun Moon Lake, Shuili, Xinyi, and ends in Tataka.

The segment from Sun Moon Lake to Tataka is a part of the unfinished New Central Cross-Island Highway (新中橫公路).

The cross-island highway plan was formally abandoned due to environmental concerns.