> What's the mechanism in place to prevent the private road owner from charging far more than the maintenance costs of the road?
Freedom of people to move their place of residence and business, and long term contracts.
Basically people don't move house willy-nilly, but when they move house they can take considerations like roads and railroads etc into account. That's what they do today, too.
Long term contracts assure that your decision will be valid for as long as you like to negotiate.
No need for all the regulation you envision here.
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However, I do agree to an extent: private companies are not a panacea by themselves. As you can see with the sorry state of eg American banking (especially historically when branch banking was banned) or in their healthcare system or what I hear about residential internet access.
What is more important is genuine competition.
Alas, adding lots and lots of layers of regulation seldom works. You just get regulatory capture instead.
In that case, you might make an argument that outright government ownership doesn't look too bad in comparison. Especially if you can have very local levels of government be responsible. Eg municipal waterworks seem to work reasonably well around the world. See eg https://en.wikipedia.org/wiki/Sewer_Socialism
What often works better is deliberately decreasing barriers to entry. Be that from foreign competition, local upstarts, or established firms branching out into other industries. (Eg Walmart tried to allow people to open bank accounts. You can guess at the lobbying storm from incumbents that caused.)
Another good measure, but extremely unpopular in practice is to make sure your regulations are as simple as possible. For example, when you have decided to privatize some government agency, say like USPS or Amtrak, you should remove all the debts from its balance sheet and then sell it strictly to the highest bidder.
In practice, that never happens. In practice, there will be a beauty contest with lots of complicated rules (eg about preserving jobs etc). Those rules give the adjudicating officials lots of leeway to decide, and thus power.
Invariably, the buyer will later on renege on their promises.
That's much harder if you just hold an auction in terms of cold, hard cash.
(I'm parroting the suggestions of 'Just Get Out of the Way' https://www.bookdepository.com/Just-Get-Out-Way-Robert-E-And... here. The wisdom of removal of debt from the balance sheet before selling wasn't obvious to me for a while: the idea is that any dollar of debt left on the balance sheet would decrease the sales price by roughly a dollar, but if the newly privatized company goes belly up, it would be politically impossible to avoid a bailout of the debt that existed before the privatization.
In practice we'd expect the new owner to add debt back on the balance sheet shortly after the sale. But that new debt will be politically much less charged.)