Britain's busiest roads urgently need £20bn of improvements over the next decade to avert gridlock for millions of motorists, according to a controversial study by the RAC Foundation.

The organisation claimed yesterday that drivers in Britain endure "the worst conditions in Europe" and demanded greater government expenditure on widening schemes and bypasses.

But critics accused it of outdated "predict and provide" thinking which would merely encourage more vehicles on to the roads and damage the environment.

The government pledged £7bn last year for a roadbuilding package. But the RAC said this was insufficient. It published a wish-list of more than 30 additional schemes.

Edmund King, the foundation's executive director, said: "Bearing in mind that roads account for 93% of all journeys, £20bn over 10 years is very little. It's half what the motorist pays in tax every year."

Environmentalists attacked the list as unnecessary. Steve Hounsham of the pressure group Transport 2000 said: "Roadbuilding on this scale is as old-fashioned and doomed as the dinosaurs. The future belongs to sustainable transport. The RAC should be supporting better public transport and better opportunities for walking and cycling."