1. A method of estimating a time-to-collision (TTC) of a vehicle with an object using a system including a camera and a processor mountable in a motor vehicle the method comprising: acquiring from the camera a plurality of images of the object at known time intervals; determining a relative scale of the object in the images; determining a function of relative velocity and relative acceleration between the vehicle and the object from the relative scale and die time intervals; and determining the time-to-collision (TTC) based on the function of the relative velocity and the relative acceleration.