That's because gas taxes are an unusually live wire in US politics, not because roads are fundamentally uneconomic. Until 2008 gas taxes did cover highway spending, and they now require general transfers because gas taxes weren't indexed to inflation. According to this website the taxes weren't increased since 1993, so in practice the US system has been deliberately starved of sustainable revenue.

https://taxpolicycenter.org/briefing-book/what-highway-trust...

"Congress could pay for projected highway and mass transit spending by simply raising the federal tax rate on gasoline and diesel fuel. Increasing those rates by 15 cents per gallon along with indexing the rates for inflation would increase federal revenues by $240 billion over ten years (CBO 2022)."

Balancing the highway books in the USA would be easy, and it's done already in most other parts of the world.

You've now admitted that most of the cost of maintaining and building roads in the US is borne by government subsidies.

I just looked up how Germany funds its road network. It turns out that 2/3rds of the funding comes from taxes (subsidies, in other words).[0]

0. https://bmdv.bund.de/DE/Themen/Mobilitaet/Infrastrukturplanu...

Funding is a subsidy only if it comes from tax/borrowing sources that aren't related to the thing being subsidized. If taxes levied on driving fund driving, that's not a subsidy. Also that page seems to be talking about all transport including railways, but we're only interested in roads here.

Germany spends ~16B EUR per year on roads at the federal level:

https://www.statista.com/statistics/1293915/federal-expendit...

The amount it gets from motor taxes is vastly in excess of that:

https://www.destatis.de/EN/Themes/Society-Environment/Enviro...

That's not surprising because Germany has the highest motor taxes in Europe:

https://www.acea.auto/press-release/motor-vehicle-taxation-b...

The US does subsidize roads but it doesn't have to, hasn't done so in the recent past and arguably shouldn't be doing so.

Germany is highly federal. The federal government only covers a portion of the cost of road construction and maintenance, with state and local governments carrying much (a majority, I think) of costs.

From what I see in your 2nd link, motor taxes do not even cover the federal government's contribution to road construction and maintenance, not to even mention what state and local governments pay.

You're saying the US doesn't have to subsidize roads, but removing that subsidy would require the US to raise fuel taxes and tolls by 200%. Rail can also break even if prices are increased. The reason governments subsidize HSR is because it has positive externalities.