Autonomous vehicles promise significant benefits to society, from reduced accident rates to greater mobility for the elderly. The biggest challenge in the design of autonomous vehicles comes from the uncertainty of the environment in which they will operate. Their control algorithms must be able to cope with driving events that occur on widely ranging time scales. For example, relaxed rural driving can accommodate planning actions every few seconds, while imminent collision avoidance requires planning and actuation on the order of a few milliseconds. Thus 'real-time' performance will imply different things depending on the context.