John Cawley and David Frisvold have become the leading authorities on soda taxes. In their current working paper with David Jones, they compare results in Philadelphia, Oakland, San Francisco, and Seattle. Philadelphia’s tax “decreased purchases by 27.7 percent,” but the authors “do not find impacts of the taxes in the other three cities combined.” Cawley has suggested these taxes should be imposed “at the state or national level, so that there’s less incentive to just drive a mile or two to evade the tax.”