Federal law prohibits banning non motorized traffic from a highway. A summary of these laws are (m)Protection of Nonmotorized Transportation Traffic.—
The Secretary shall not approve any project or take any regulatory action under this title that will result in the severance of an existing major route or have significant adverse impact on the safety for nonmotorized transportation traffic and light motorcycles, unless such project or regulatory action provides for a reasonable alternate route or such a route exists. Keep in mind the United states Code is not the law it is the Cliff notes to the readers digest condensed version of the law. So I'm going to offer the following with out proof. The federal government a state or a county has the authority to move a highway. The new route must be within 2 miles of the original route and be of equal or greater quality than the original route. Therefore if a detour for non motorized traffic exceeds this then it is not legal. If an interstate is built within the right of way of a pre existing highway then a state can not ban non motorized traffic on that section of the road. A state law that is in conflict with a federal law can not be enforced and may be unconstitutional. A recent ruling in Louisville is relevant here link below. Here are several roads that I think are in violation of this law. Interstate 5 on the northern California border. All interstates in Arizona. They allow bicycles but prohibit pedestrians. Interstate 25 on the northern New Mexico border. Interstate 72 on the western Illinois border. Interstate 69 where it blocks highway 57 in Indiana. Interstate 20 where it runs over highway 78 in Alabama. In addition any highway that is shut down for maintenance must provide a detour route even if such a detour route runs across private property or an interstate. The detour route then legally becomes the highway. Again non motorized traffic can't be banned on this detour.
https://www.wdrb.com/in-depth/louisville-judge-rules-kentucky-speed-limit-laws-unconstitutional/article_41bee7d0-db1c-11e9-90c3-4f53828e8df3.html