Vehicle automation is coming, but environmental and energy imperatives are NOT what's motivating it. In fact, its energy and environmental outcomes are deeply uncertain. The promise of autonomous vehicles (AVs) is greater safety and mobility. The question is: How do we achieve that promise while reducing greenhouse gas (GHG) emissions and saving energy? Future AV scenarios range widely, from dramatically higher to dramatically lower GHG emissions. Fortunately, the best case for the environment is also the best case for business, for the economy, and for safe and affordable mobility: autonomous Taxi fleets that operate clean vehicles, encourage ridesharing, and are integrated with other public and private transportation modes. Governments, businesses, drivers and riders make decisions that can increase or decrease the likelihood of the best-case scenario. The stakes are very high, as the transportation sector is now the largest source of U.S. GHG emissions. Emissions are equal to (1) emissions per-vehicle-mile, multiplied by (2) vehicle-miles traveled (VMT). This paper explores the business, technology and behavioral decisions with respect to automation that have the greatest impact along these two dimensions. It then identifies policy interventions that could influence these key decisions to achieve the best GHG outcomes. AVs are an important international issue, although this paper is focused on the United States.