Autonomous vehicles (AVs) are conveyances to move passengers or freight without human intervention. AVs are potentially disruptive both technologically and socially 1-3 , with claimed benefits including increased safety, road utilization, driver productivity and energy savings 1-6 . Here we estimate 2014 and 2030 greenhouse-gas (GHG) emissions and costs of autonomous taxis (ATs), a class of fully autonomous 7,8 shared AVs likely to gain rapid early market share, through three synergistic e ects: (1) future decreases in electricity GHG emissions intensity, (2) smaller vehicle sizes resulting from trip-specific AT deployment, and (3) higher annual vehicle-miles travelled (VMT), increasing high-e ciency (especially battery-electric) vehicle cost-e ectiveness. Combined, these factors could result in decreased US per-mile GHG emissions in 2030 per AT deployed of 87-94% below current conventionally driven vehicles (CDVs), and 63-82% below projected 2030 hybrid vehicles 9 , without including other energy-saving benefits of AVs. With these substantial GHG savings, ATs could enable GHG reductions even if total VMT, average speed and vehicle size increased substantially. Oil consumption would also be reduced by nearly 100%.Many automakers and Google plan to rapidly commercialize AVs (refs 4,8,10), although it will take time to gain widespread market share. AV functionality ranges from lane-keeping and parking assistance features to full control without human input 7 . As of 2014, four US states and Washington DC allow AV testing on roadways, with thirteen more contemplating similar laws; Nevada is the first state offering 'certificates of compliance' for non-testing use of AVs (ref. 4). For more background information, see Supplementary Note and Supplementary Table 1. The US Energy Information Administration (EIA; ref. 11) projects GHG intensity decreases between 2014 and 2030 in gasoline (3.8%) and electricity (8.5%), due to growing renewable energy contributions. However, GHG policies may lower intensities further. The US Environmental Protection Agency (EPA) has proposed a rule to lower average GHG intensity of electricity 30% by 2030 (ref. 12), whereas in California (CA) GHG electricity intensities may fall 55% by 2030 as a result of several policies 13 . We considered GHG intensities of gasoline and electricity based on 2014 and 2030 EIA projections, and 2030 GHG electricity intensities from EPA and CA (applied across the US). Also considered were GHG emissions for hydrogen produced from natural gas reforming, water electrolysis or other methods 14 ; the former two were estimated using GHG energy intensities from EIA for natural gas, and EPA and CA for electricity.Combining GHG energy intensities with vehicle technology efficiencies produced a wide variety of GHG emissions intensities per mile. Passenger car and light truck fuel efficiencies were combined using fleet mix ratios projected for 2014 and 2030 (ref. 11). As shown in Fig. 1 (see Supplementary Table 2 for additional data), there is a 52% decrease in GHG emis...