This paper explores how advanced transportation technologies and alternative fuels, together with policies aimed at reducing greenhouse emissions and energy use, can help address the energy challenges facing the US transportation system over the coming decades. Three scenarios of future transportation energy use are constructed to represent alternative views of the urgency with which the federal government and the American people will view these challenges, and the policy interventions they will seek. The analysis concludes, first, that US transportation energy use and greenhouse emissions will increase virtually unchecked without policy intervention or unforeseen large, long-lasting fuel price increases, and second, that policies that stimulate technology development and deployment can substantially slow this growth-though not without a considerable time lag due to the slow turnover of the vehicle fleet and limits on the rate of technology development and deployment.