Binocular information about time to collision and time to passage

It is well known that, when an object's horizontal relative disparity is changing appropriately, most observers report a compelling impression that the object is approaching and will collide with the observers at some future instant. Here I derive a new equation, namely TTC approximates (ddelta/dt)/(d(2)delta/dt(2)). This equation relates TTC to retinal image variables without involving a knowledge of the approaching object's distance or speed. In this respect the new equation is the binocular equivalent of the well-known equation for tau.