Switzerland could see kilometre-long traffic jams at its borders if voters back the People's Party's "No to 10 million" initiative on 14 June.
A new Ecoplan study says approval would push the country out of Schengen, forcing France, Germany, Italy and Austria to reintroduce checks at every crossing.
Geneva, Ticino and Basel would be worst hit, with more than 400,000 cross-border workers commuting each day.
Peak-hour waits of more than an hour are conceivable. In the worst case, two-thirds of cross-border staff could give up their Swiss jobs, hitting hospitals and services hardest, and shaving up to 3.9% off GDP.
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