Voters will get the final say on a major autoroute expansion which includes an extra lane on the A1 between Geneva and Lausanne.
Anti-roads campaigners say the cost – at over CHF 5bn – is too much and the project goes against plans to cut the country’s carbon emissions.
As it’s a referendum, only 50,000 signatures were needed – but 100,000 have been submitted with a week to go until the deadline.
Parliament had given to go ahead at the end of last year.