Estimates by Halcrow Consultants revealed there could be a 46% increase in traffic by 2031, from 148 billion vehicle kilometres a year to 216 billion.
Although the existing network could cope with some increase, there would need to be enough capacity to cope with another 55 billion vehicle kilometres a year.
The £70 billion would be split between new motorways, widening motorways, and nearly 400 miles of new trunk roads. But spread over 29 years, it equates to £2.4 billion a year extra spending, more than twice the current level of investment in the roads, or a 50% increase in roads spending in the 10 Year Plan for twice as long.
Although this may seem a large increase, consultant David Bayliss, visiting professor in transport studies at Imperial College, London, said it was just half the amount allocated to rail investment in the Government's 10-Year Plan.
Transport Secretary Alistair Darling said: 'In 2004 we will review the 10 Year Plan and the amount we spend on roads or rail. There is a legacy of under investment in the roads.'