1. A method of estimating a time-to-collision (TTC) of a vehicle with an object comprising the step of: (a) acquiring a plurality of images of the object at known time intervals between the times t at which the images are acquired; (b) determining a momentary time T v (t), which is equal to an instantaneous distance between the vehicle and the object at time t divided by an instantaneous relative velocity, from information derived from the images and the time intervals; and determining the time-to-collision (TTC) based on a rate of change over times t of momentary time T v (t).