Title 23 Section 109m United States Code.

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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

[John Nettles]

Out of curiosity, where did you come up with the 2 mile limit?  I always heard it was riders could ride a prohibited road "unless reasonable alternatives are available".  The question then becomes what is reasonable.  The very old school of thought, i.e. 80s/90s, was a detour over 25 miles extra, not 2.  I like yours better of course. 

That said, IF a reasonabe alternative IS available, I would prefer to ride it anyway.
Tailwinds, John

[Westinghouse]

I have cycled interstates many times. Signs indicate where bicycles enter and exit. It's shorter and faster than the mapped bicycle routes. It has its downsides that may far outweigh any benefit you might have gained versus going mapped routes.

[froze]

My understanding of the law is if at a on ramp to a freeway and there is a sign posted that says "no bicycles permitted" (could be worded many different things to mean the same thing) then you can be on that part of the highway; if there is no sign then you are permitted to ride until you see a sign on the freeway at an off ramp that says "all bicycles must exit" then you have to get off.

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Sorry for the delay I've been out riding my bicycle. A county a state or the federal government is allowed to move a highway. The new route must be of equal or greater quality of the original route and be within 2 miles of the original route. Any thing more that this requires an act of congress. All of this is straight forward and any highway department should have this information. This is what I mean by the US code being the Cliff notes of the Readers Digest condensed version of federal law. I'v decided to include all interstates in North Dakota to the list. They allow bicycles but not motorized bicycles. The first motorcycles were motorized bicycles and I think the federal definition includes them. The problem is some internally geared hubs look like motors from a distance and people are certainly being pulled over for this. When the technology becomes available I will be using regenerative braking. This turns a generator into a brake which dumps the power into a battery. The battery then turns the hub into a motor and allows a motorized assist. I will use this not because I'm lazy but because lithium batteries can not be overcharged. It is best to keep them under charged by using them as an assists. Of course at night you can discharge them into your lights.