23 CFR 658.21 - Identification of National Network.
(a) To identify the National Network, a State may sign the routes or provide maps of lists of highways describing the National Network.
(b) Exceptional local conditions on the National Network shall be signed. All signs shall conform to the Manual on Uniform Traffic Control Devices. Exceptional conditions shall include but not be limited to:
(1) Operational restrictions designed to maximize the efficiency of the total traffic flow, such as time of day prohibitions, or lane use controls.
(2) Geometric and structural restrictions, such as vertical clearances, posted weight limits on bridges, or restrictions caused by construction operations.
(3) Detours from urban Interstate routes to bypass of circumferential routes for commercial motor vehicles not destined for the urban area to be bypassed.