The advent of autonomous cars creates a range of new questions about road safety, as well as a new collaborative domain for CSCW to analyse. This paper uses video data collected from five countries - India, Spain, France, Chile, and the USA - to study how road users interact with each other. We use interactional video analysis to document how co-ordination is achieved in traffic not just through the use of formal rules, but through situated communicative action. Human movement is a rich implicit communication channel and this communication is essential for safe manoeuvring on the road, such as in the co-ordination between pedestrians and drivers. We discuss five basic movements elements: gaps, speed, position, indicating and stopping. Together these elements can be combined to make and accept offers, show urgency, make requests and display preferences. We build on these results to explore lessons for how we can design the implicit motion of self-driving cars so that these motions are understandable - in traffic - by other road users. In discussion, we explore the lessons from this for designing the movement of robotic systems more broadly.