Provided is a visual vehicle speed estimation system based on camera output and calibration for such a vehicle speed estimation system. The calibration allows use of the system where the absolute camera position is unknown. Calibration determines an absolute-position-independent relationship between an image space placement and a physical space position relative to the camera. The calibration is done on the basis of camera output using vehicle features of known dimensions and some assumed physical constraints related thereto to provide a conversion relationship between image coordinates and physical space coordinates in a physical space defined in relation to the camera. This relationship is then used to estimate vehicle speeds based only on the visual information provided by the camera. Abstract is not to be interpreted as limiting.