capturing, with a camera having an optical axis aligned with a road environment, a plurality of images of a calibration vehicle traveling relative to the optical axis within the road environment at a known ground speed;

locating a calibration image feature in an image of the calibration vehicle, wherein the calibration image feature is located with respect to a vanishing point defined in an image plane of the camera where vehicle trajectories in a field of view of the camera converge;

computing an optical flow of the calibration image feature between two or more image frames; and

determining, based on the optical flow of the calibration image feature, a model that relates an image speed of the calibration image feature and the known ground speed of the calibration vehicle,

wherein the model comprises a plurality of parameters, and wherein said determining the model includes determining the plurality of parameters of the model, wherein the plurality of parameters of the model include: a proportionality constant C relating ground speed V to image speed v, image coordinates x0 and y0 of the vanishing point in the image plane, and a horizon slope factor a related to camera roll of the camera about a horizon of the road environment.