S.D. Codified Laws § 10-12-16
Any unexpended balance remaining in any road, highway, or bridge fund of the county may, at the end of the fiscal year be transferred to the county highway and bridge reserve fund established in § 10-12-13. Any money in said fund, not necessary for the purpose of matching federal aid grants heretofore made or for the current year may be expended for laying out, marking, constructing, reconstructing, and maintaining any roads and any bridges on the county secondary federal aid system under the jurisdiction of the board of county commissioners, or such funds may be allowed to accumulate as a reserve fund and be used for future improvement and construction of public highways and bridges. No part of such fund shall revert to the general funds of the county nor shall any of said funds be used for any other purpose than for highway or bridge purposes as provided herein.
SDCL 10-12-16