This paper is concerned with the detection and tracking of obstacles from a camera mounted on a vehicle with a view to driver assistance. To achieve this goal, we have designed a technique entirely based on image motion analysis. We perform the robust estimation of the dominant image motion assumed to be due to the camera motion. Then by considering the outliers to the estimated dominant motion, we can straightforwardly detect obstacles in order to assist car driving. We have added to the detection step a tracking module that also relies on a motion consistency criterion. Time-to-collision is then computed for each validated obstacle. We have thus developed an application-oriented solution which has proven accurate, reliable and efficient as demonstrated by experiments on numerous real situations.