The benefit in this case potential, so it's hard to quantify it unless and until it is realized. But if it comes to that, it would be absolutely crucial for national defense.

I don’t think the analysis is that difficult. Imagine that the interstates were only used for military transportation (both for emergencies, which presumably haven’t happened much, and for regular logistics). Surely the ongoing maintenance costs would be vastly reduced.

Those costs would be the costs that “we the people” accept as valid usage of taxpayer money. You could throw in costs for other public interests like emergency logistics (e.g. FEMA moving goods and personnel for hurricane relief), first-class mail, etc., and you’re still probably only accounting for a very small portion of the taxpayer money that is currently going toward regular interstate maintenance.

The rest of the taxpayer costs are just subsidies of private interests, namely commercial freight and personal transportation. Those subsidies aren’t “free.” They come at the expense of other private interests, like freight and passenger railroads. These subsidies undoubtedly drastically change the economy, pushing everything toward just-in-time fulfillment, increasing greenhouse gas emissions, etc.

I would like to see these private subsidies reduced or eliminated, perhaps by charging tolls.

Practically speaking you’d be shifting the burden from the people who can afford it, to people who can’t. I see similar arguments from people who resent paying into social security and other things. It seems to be a combination of ideological convictions, lack of empathy, and unawareness of why these kind of measures were created in the first place. Ignoring all of rhe knock-on effects of a massive road network and it’s upkeep, it’s ability to reduce the cost of freight, and subsequent lack of rioting from people who couldn’t afford milk without those “subsidies” seems ultimately short-sighted.

I’m not quite convinced that the effects would be so regressive, especially in the longer term. Couldn’t most national food transportation (including perishables like milk) be done with rail freight? Even if the retail costs stayed the same, surely the environmental costs would be drastically reduced (and surely the longer-term costs of climate change will be extremely regressive).

In other words, I question the premise that federal interstates reduce the cost of freight if you include the environmental externalities.

Ok, that’s a good question to raise, but maybe you should move beyond asking the question and try answering it. You could learn about how different shipping methods are used, how they interact, and just how extensive a rail network a country as massive as the US would require to replace roads. You could also consider that withnthe death of those roads, you’d be utterly snuffing out smaller rural communities, which would be too small or too distant from major population centers to justify a massive rail project.

All in all, if you have such a strong proposition about not paying for roads, it seems like you should already know these things. Now, could roads be replaced? Probably, at enormous cost both financial and environmental. Building massive rail networks is not quick or cheap, you’re talking about megatons or steel and iron being welded after all.