Transport energy use and carbon emissions continue to rise, but both need to be drastically reduced. Conventional proposed solutions, all already used to some extent, include a shift to low carbon transport fuels, major improvements in vehicular fuel efficiency, and modal shift. However, their impact has been marginal. This paper instead examines the extent to which fully automated vehicles could contribute to the environmental sustainability of global passenger transport. Fully automated vehicles were found to lead to either an increase or, at best, a slight decrease in energy use and greenhouse gas emissions, and so will be of marginal use at best for reducing emissions in a business-as-usual world. Reasons for this conclusion are first, their potentially lower time and money costs would tend to increase vehicular travel, offsetting any energy efficiency gains, and second, that they face serious problems that could delay or even prevent their widespread introduction.