Nine Northeastern states and the District of Columbia committed yesterday to forming a cap-and-trade system for the transportation sector, embarking on one of the most ambitious regional programs ever undertaken to cut carbon dioxide. Lab's Kate Larsen of Rhodium Group weighs in on the magnitude of this announcement for US emissions, with transportation the largest source of carbon dioxide for the third year running. She says this is "significant, because it tackles transportation in a new way that goes beyond relying on federal authority and California’s approach, which other states have replicated and relied on. California has special authority under the Clean Air Act to set vehicle emissions standards.