Like Marek said above about education and health care.
I mean, come on, we're talking about roads here, without which you can't bike from A to B. When the capital development of a country becomes a by-product of the activities of a casino, the job is likely to be ill-done. — John M. Keynes
This why I don't like 'markets should...' arguments. There's no clean definition of social necessity.
What markets really do is act as amplifiers of political positions. They're not designed to set prices rationally, they're designed to make some people rich and powerful and other people poor and irrelevant.
Is there anything at all which markets do which can't be done better in other ways?
As Sassafras said, reality isn't quite that tidy.
And if there's no other capacity on alternative modes of transport, trucking isn't key infrastructure?
What should also be done is looking at how the transport infrastructure should be shaped for the future. I think it is plausible that we will see a scenario with much more rail transport, with the final leg being performed by relatively small electric trucks with a limited autonomous range (but also by for instance coopting tramways in cities).
Building big new highways (as Zapatero is doing) will mainly favour mule power...
