N.D. Cent. Code § 24-01-05
The director may designate, from time to time, those roads selected under section 24-05-16, as the county highway system not exceeding twenty-two thousand five hundred miles [36210.24 kilometers] in length on which federal aid funds must be expended as may be provided by such appropriations. In designating such system, the director may remove from the state highway system those parts which are low in standard of improvement and type of traffic service and which will be released from maintenance agreement or agreements with the federal government. No mileage on the state highway system may be placed on the county road system without the consent of the board of county commissioners of the county in which the road lies. The director may enter into an agreement with the board of county commissioners of any county providing for the transfer of highways from the state highway system to the county road system of such county.
N.D.C.C. § 24-01-05