In the past few years, many cities have attempted to mandate the use of hybrid taxicabs. The taxi industry, arguing that the Energy Policy and Conservation Act and the Clean Air Act preempt such mandates, has successfully opposed them. Mandating hybrid cab use, however, is but one important aspect of a larger push by states and local governments to enact progressive environmental legislation and policies with greater breadth than those of the federal government. An example is California’s battle to enact greenhouse gas emissions regulations and its conflict with the Environmental Protection Agency. Though New York City and Boston lost on preemption grounds, their attempts are important in the context of the battle over environmental federalism. In the long term, the Energy Policy and Conservation Act should be amended to allow for more progressive environmental regulations at the state and local level.