SH 321 is part of the eastern Greater Houston area evacuation route, providing a corridor for communities such as Baytown, Kemah, and Pasadena.

SH 321 continues northward through the neighborhoods of Dayton as a 4-lane highway, eventually changing to a 2-lane rural road, then has 2 directional overpasses over the Union Pacific Beaumont subdivision rail line.

The road continues north through rural Liberty County and meets again with the northern terminus of FM 1008.

As it joins SH 105, the route turns west then northwest as it approaches the city of Cleveland, where is it known as Houston Street.

The Houston-Galveston Area Council long term plans (by 2025) show plans to expand SH 321 to an urban four lane divided highway with left turn lane, south out of Cleveland until SH 105 cuts east (6–8 miles south of Cleveland).