LONDON — Luxembourg is a small country with big traffic jams.

So when Prime Minister Xavier Bettel was sworn in for a second term on Wednesday, his governing coalition promised free mass transit for all, which would make the country the first to offer such a benefit.

Luxembourg is barely larger than a city-state, with a population of about 560,000 — roughly equivalent to smaller European capitals like Copenhagen. But more than 180,000 workers commute across the border from Belgium, France and Germany. The average salary, of just over 50,000 euros (or $56,000), is almost 40 percent higher than in France, for example.

“It’s basically like a city which has suburbs abroad,” Olivier Klein, a researcher at the Luxembourg Institute of Socio-Economic Research, said by phone on Thursday.

Part of the problem is that Luxembourg already has the highest number of cars for its population in the European Union: 662 for 1,000 people, bringing it closest in the region to the United States, a world leader with more than 800 cars per 1,000 people.