It’s a long-term thing, like good primary education. Get 'em into good habits while they’re young. Probably work great if kids have bus passes for school most of the year and so get into the habit of using public transit, then suddenly need to either pay or exercise over the summer break

This of course assumes a robust transit system, making the idea of taking the bus or train somewhere even worthwhile in the first place. Some places have that, some not so much