This paper exploits the potential contribution of autonomous vehicles towards sustainable urban transport through their interaction with conventional traffic. Anticipating the gradual increase in autonomous vehicles use, the research examines how the adoption of a non-aggressive, environmentally friendly driving style from a varying percentage of the traffic may influence the driving style of the rest of the traffic, taking into account, system capacity, vehicle performance and user comfort indicators. Although autonomous vehicles can be powered either by conventional internal combustion or electric engines, the contribution to urban transport sustainability examined in this research concerns the assessment of the potential environmental benefits that might arise from mixed traffic streams (driverless and conventional). The research uses microscopic traffic simulation to assess the performance of mixed traffic streams. The environmental performance of the integrated traffic flow is assessed using instantaneous vehicle emission modelling for different levels of autonomous vehicle market penetration.