Why would you have private ownership of a road?

I recently had an interesting discussion on PPPs and the newest (and most controversial) Sydney tunnel. It got me thinking about its implications for economics and politics. What are the issues to consider if we are honestly trying to be non-ideological in the discussion of privatised roads and PPPs? These appeared to me to be the issues requiring consideration.

Are there management benefits of running a road under private ownership? Is there any more involved than just letting a road sit there, be maintained, and collect tolls?

Can a road be seen as in competition with other roads (or means of transport)? And related, is competition a requirement to ensure public good is realised by private ownership. Are there such things as natural monopolies? Are natural monopolies recognised in our current view of modern economics, and, if so, how are they supposed to be managed?

[The new Sydney tunnel has seen associated road closures to support more tunnel use.] Government-owned roads could still be supported by closures of other roads, but would these decisions be made on just profit-related grounds, or additionally on other grounds, eg, equity, environmental, efficiency? (We can assume any concessions on roads originate from requests/demands by profit-seeking private owners).

Benefits from competition in construction and management are expected. A government can obtain these benefits while still owning the road.

Private ownership requires 1/ higher rates of interest on monies borrowed to build the road, and 2/ profit to the owner/shareholder. To what degree does this benefit the public? To what degree does this just transfer wealth? From whom to whom?

Tolls are user pay systems. User pays is to some degree a flat tax system, but it does support economic efficiency. Tolls can still be used on government-owned roads.

What benefit is there in removing a road from a government budget to the taxable, private economy? Is this a real-world benefit or a paper-based benefit (higher GDP etc), or a benefit at all?

To what degree do PPPs and private ownership of roads affect corruption? Does private ownership place roads outside political manipulation, corruption, etc? Alternatively, does the big money involved lead to other forms of corruption?

Who covers the risk? Is this maybe the major benefit from private ownership? If so, is it realised in general PPPs and in these contracts?

Is there ideology in Private-Public Partnerships, as displayed in road ownership issues? Is ideology evident in immediate decision making on specific cases? Probably not: it’s more likely just poor thinking in the development of specific contracts. More broadly, is ideology evident in the underlying assumption of the benefits or superiority of PPPs? This involves examination of the history of economic thinking on natural monopolies. We need an economic historian to judge this one.