A natural gas powered North County San Diego transit bus travels through Encinitas, California June 26, 2015. Fifteen years ago, California led the way to cleaner transit buses with strict tailpipe emissions standards that effectively ushered out diesel as the primary fuel for buses in the state and replaced it with natural gas. Now, California is poised once again to take the lead, this time by mandating a switch to so-called “zero-emission” buses by 2040. (Photo by Mike Blake/Reuters)
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