A system for passively determining a speed of moving vehicles includes a single imaging device (103) for acquiring a plurality of images at a first time and at least at one second later time, wherein each image includes a moving vehicle (406) on a vehicle pathway (402) and at least one fixed object (404) proximate to the vehicle, defining a first plane (π1) intersecting an adjacent portion of the vehicle pathway (402), wherein no geometric lengths of the vehicle (406) are known a priori. The system can also include a processor (121) operable to receive data associated with the images and determine a speed of the vehicle (406) in the images, wherein the processor (121) estimates at least one geometric length of the vehicle (406) by applying homography using the plurality of images, and where the processor (121) determines a speed of the vehicle (406) from the plurality of images and the estimated geometric length using a velocity estimation method.