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