S.D. Codified Laws § 31-10-8

Any bridge purchased or constructed under §§ 31-10-2 to 31-10-11, inclusive, shall be free from tolls. However, tolls may be collected on the traffic crossing the bridge until such time as the net sum of tolls so collected plus funds paid as provided in §§ 31-10-6 and 31-10-7, equals the cost of the structure plus a reasonable rate of interest on deferred installments of the cost. The portion of the net tolls collected on the bridge, going to the State of South Dakota shall be credited to the state highway fund.