I'm not familiar with how things are done in Europe, but in the US fuel taxes aren't enough to pay for road maintenance, let alone new construction and externalities like pollution. New construction is typically mostly done with federal grants (newly printed money) and pollution we all just breathe.

According to the Swiss government that's actually the case there: https://www.bfs.admin.ch/bfs/en/home/statistics/mobility-tra...

"Degree of coverage of motorised road traffic infrastructure costs: 111%"