The topic of fare-free transit is hot. Public transit is a public good and it’s also good for the environment, economic opportunity, social mobility, traffic congestion.

Various places around the country are piloting the idea: Boston Mayor Michelle Wu has pushed for a free bus system, and Kansas City eliminated bus fares. In California, several bills in the legislature would create free transit for youth, and some local agencies aren’t waiting for them to pass. Temporary fare-free programs at agencies around the state have been supported by cap-and-trade grants, with the expectation that more people riding means less people driving, and fewer greenhouse gas emissions.

Some have argued that free transit can’t work, citing a variety of reasons. As Stephen Dubner points out on his recent podcast for Freakonomics Radio, “it’s complicated.”