Transport and infrastructure minister Melanie Schultz has based her decision to widen many of the country’s motorways on unrealistic traffic growth forecasts, according to research group CE Delft.

The company was asked to assess the minister’s calculations by several environmental groups. In the report, CE Delft claims the need for and benefits of new roads have been over-estimated and based on mistaken priorities.

CE Delft looked at six motorway projects with a budget of some €5bn. It concluded the minister’s projects were based on forecasts which have since been overtaken and used the maximum growth predictions.

It compared actual traffic growth with scenarios developed for new roads. It concludes that far fewer cars will actually use roads than forecast, pointing out that traffic volumes have decreased since the start of the crisis.

The minister’s motorway plans are due to be discussed in parliament on Monday.