1. A method of estimating a time-to-collision (TTC) of a vehicle with an object comprising the step of: acquiring a plurality of images of the object at known time intervals; determining a relative scale of the object in the images; and determining the time-to-collision (TTC) based on the relative scale and the time intervals, wherein the time-to-collision (TTC) is responsive to the relative velocity and the relative acceleration between the vehicle and the object.