S.C. Code Ann. § 57-5-10: Composition of state highway system in general.

The state highway system shall consist of a statewide system of connecting highways that shall be constructed to the Department of Transportation's standards and that shall be maintained by the department in a safe and serviceable condition as state highways. The department may utilize funding sources including, but not limited to, the State Non-Federal Aid Highway Fund and the State Highway Fund as established by Section 57-11-20 in carrying out the provisions of this section. The complete state highway system shall mean the system of state highways as now constituted, consisting of the roads, streets, and highways designated as state highways or designated for construction or maintenance by the department pursuant to law, together with the roads, streets, and highways added to the state highway system by the Commission of the Department of Transportation, and the roads, streets, and highways that may be added to the system pursuant to law. Roads and highways in the state highway system are classified into three classifications:

(1) interstate system of highways;

(2) state highway primary system; and

(3) state highway secondary system.