N.D. Cent. Code § 24-01-36

While the necessary bridges on any state highway must be construed and considered a part of such highway, nevertheless, such bridges may be designed, erected, and contracts awarded separately therefor, and such bridges may be designed, erected, and contracted irrespective of the time when the highway contiguous thereto has been or may have been improved. All necessary fills and approaches to any bridge must be construed and considered as part of such bridge.