With an aging, dwindling population, and polluted air, something needed to change in Dunkirk, France. So when Patrice Vergriete became mayor of the post-industrial city by the sea in 2014, he promised to make public transportation totally free.

Vergriete foresaw multiple benefits: Poor citizens who were already riding buses would save cash. Seniors could have an easier time making social connections. Families with cars who had never thought to ride transit to parks and stadiums would be prompted …