1. A system for passively determining a speed of moving vehicles, comprising: an imaging device for acquiring images of a moving vehicle on a vehicle pathway and a fixed object proximate to the moving vehicle, the fixed object defining a first plane that intersects an adjacent portion of the vehicle pathway, wherein no geometric length of the vehicle is known a priori; and a processor operable to receive data associated with the images and determine a speed of the moving vehicle in the images, wherein the processor estimates a geometric length of the vehicle based on applying homography using the images, and wherein the processor determines a speed of the moving vehicle from the images and the estimated geometric length using a velocity estimation method.