What does "using" public transit mean, according to the data you're using?

I notice that if you increase the minimum percentage of the population that uses public transportation to 15 percent, it eliminates basically everywhere except the major east coast cities, Chicago, and San Francisco.

I wouldn't find that surprising if using public transportation means using it exclusively, but I would find it surprising (and disheartening!) if it means using it regularly, or at least some of the time.