The Bay Area's recent six days of free public transit during hot, smoggy summer commutes were popular, but whether the Spare the Air program justified its hefty price tag is up for debate. The Metropolitan Transportation Commission, the region's transit planning and funding agency, spent about $13 million to compensate transit agencies for lost fare revenue during those six days in June and July. Yet there is no clear way to measure whether the increase in transit ridership resulted in less air pollution. One goal of the program -- getting more people to ride public transit -- clearly worked. Every...