There are two arguments for road privatization that are plain daft.
One is that it will “unlock large-scale private investment” from pension and sovereign wealth funds. This argument fails because the government can already borrow from these through the gilt market at almost zero cost - the yield on 50-year index-linked gilts is below 0.2%. The cheapest way to invest in roads is for the government to borrow. To think otherwise is merely irrational debt phobia.
The second argument is that the private sector can manage the roads better than the Highways Agency. This fails because, insofar as the private sector is more efficient than the public, it is because the pressure of competition forces it to be so. But giving private firms long-lived leases over natural monopolies is no way to increase competition.
So, what is the case for privatization? Jonathan sees it as a step towards road pricing. But there is another argument.
It originates in perhaps the most important fact about western capitalism in recent years - that there is a dearth of investment opportunities; this is why business investment as a share of GDP has fallen in recent years, and why companies have for a long time been saving much more than they‘ve invested in real assets.
Faced with this problem, one function of the state is to create new investment opportunities. And road privatization - like steps towards NHS privatization - does just this.
This is, of course, nothing new. In the 50s and 60s, the state helped create profitable investments by maintaining aggregate demand through increasing public spending - in the US especially upon the military. In the 80s, it did so by attacking workers’ bargaining power and thus relieving the profit squeeze. Although the precise form in which the state helps foster private sector profits and investment varies according to circumstances (and maybe ideology)*, the fact is that a key function of the state in capitalist economies is to help create profit opportunities. As Paul Sweezy wrote in one of the classics of Marxist economics:
We should naturally expect that the state power under capitalism would be used first and foremost in the interests of the capitalist class since the state is dedicated to the preservation of the structure of capitalism. (The Theory of Capitalist Development, p248)
* There’s no inconsistency between profits being raised sometimes by full employment policies and at other times by engineering recession. If we’re in what Marglin and Bhaduri call a stagnationist regime, the former works, but if we’re in an exhilarationist regime, the latter does so. During the 70s, the UK moved from the former to the latter, necessitating - from capitalists’ point of view - different policies.