By Richard Scott
Transport correspondent, BBC News
The Bingley relief road was one of the schemes the report assessed
The benefits of major road building schemes in England are being overstated by the government body responsible, a report has said.
The Campaign for Better Transport says congestion, noise and air quality usually end up worse than predicted.
It argues that without better modelling it is impossible for residents, public inquiries and ministers to decide whether a new road is worth building.
The Highways Agency says the schemes in the report do provide value for money.
In its report, the campaign says that until the agency provides more accurate forecasts, there should be a freeze on major road building projects.
Decisions on whether to invest in major A-road and motorway schemes are based on analysis by the Highways Agency.
It predicts what the economic benefits will be, what will happen to congestion and the effect on the environment.
But the Campaign for Better Transport's study says these forecasts are not reliable, with the results generally worse than predicted.
It found that, overall, traffic increased substantially after road building, but that the increase was usually under-estimated.
"These reports should set off alarm bells for anyone contemplating major road building and for taxpayers who will eventually have to pay for them," said Richard George, roads and climate campaigner for the campaign.
"In difficult economic times, taxpayers must know that their money is being well spent. Instead, they're being fobbed off with half-baked calculations which are little better than guesswork."
The study found that bypasses often simply move the congestion from one community to another.
Residents also complained that if a bypass did successfully move congestion out of a village, some drivers would then use the village as a "rat-run", driving too fast along its roads.
The Bingley relief road in West Yorkshire is cited as a "particularly poor example of forecasting".
The study says the case for the new road was significantly overstated because of basic modelling flaws, with nearby traffic-generating schemes included in the argument for the new road when they had already been cancelled.
In contrast the modelling of the A11 at Roundham Heath did not include nearby traffic-generating schemes which had already been built, so underestimated the eventual congestion.
The Highways Agency insists the schemes gave value for money
And the report claims the Highways Agency's own consultants conclude that forecasts of economic benefits are "generally not accurate".
Increased traffic levels usually mean increased pollution and noise as well.
The campaign argues that all of this makes it impossible to decide whether it is worth spending hundreds of millions of pounds on a project, and it wants the agency to learn from its mistakes and produce more accurate forecasts.
The Highways Agency said the schemes looked at by the Campaign for Better Transport did give value for money, and have improved congestion, safety and journey times.
A spokesman said: "We use the latest data and traffic modelling methods available to us.
"However we are not complacent and we are constantly working to improve our assessment of all the aspects that feed into scheme appraisals."