Cameron's tall order in selling off Britain's roads

Privatisation of the railways was one step too far even for Margaret Thatcher as she raced to sell off state-owned industries in the 1980s.

So what would she make of the plans outlined by David Cameron today to sell off the roads?

At the heart of the proposals is a plan to leverage more private sector money into the maintenance and construction of our road network.

The part of the plan involving new roads is perhaps easier to understand. Much like the M6 toll in the West Midlands, a private company builds the road and then recovers the money and pays for the upkeep by charging drivers to use it.

The Prime Minister will say improvements to the A14 "could be part-funded through tolling."

Less clear is how a private company would pay to maintain an existing road - as we are assured new tolls on these routes would not be an option.

The government wants large-scale private investment to come from sovereign wealth funds and pension funds.

They could be paid a proportion of the Vehicle Excise Duty - but private firms would have to make this tax go further by spending it more efficiently than the Highways Agency. And they would want to make a profit on the contact.

It is a tall order and the attraction is not immediately obvious.

Drivers consistently say they pay enough tax and many will conclude the government should spend it on road schemes rather than give it to private firms looking to make money.