1. An obstacle detection method, comprising: (a) provisioning a vehicle with a camera having an external field of view, the camera having an imager defining an image plane having a vertical aspect and a horizontal aspect, wherein the imager comprises an array of sensing pixels; (b) acquiring a set of image frames captured by the camera while the vehicle is in motion; (c) determining an object present in the set of captured image frames by edge detection; (d) determining a relevancy of the determined object by (i) selecting first and second feature points of the determined object that are spaced apart vertically in a first captured image frame, (ii) tracking positions of the first and second feature points of the determined object over at least a second captured image frame, and (iii) determining that the determined object is a relevant object responsive to a determination that the first and second feature points move dissimilar distances in physical space between the first and second captured image frames and within a tolerance; (e) estimating motion of determined relevant objects relative to the vehicle to determine if any determined relevant object is likely to become an obstacle to the vehicle; wherein step (d)(iii) is determined by reference to the image plane and, where the first feature point is vertically lower than the second feature point, and wherein the method includes: estimating, from the first captured image frame, a physical distance D 1 of the first feature point O 1b to the camera in a selected physical horizontal plane; estimating, from the first captured image frame, a physical location D T of the second feature point O 1T assuming that the second feature point physically lies at a height T above the selected physical horizontal plane; estimating, from the second captured image frame, a physical distance D 2 of the first feature point O 2b to the camera in the selected physical horizontal plane, and determining the distance ΔD between D 2 and D 1 ; estimating a physical location D* T which is equivalent to D T +ΔD, with D* T being located at a height T above the selected physical horizontal plane; computing a nominal feature point coordinate O* 2t in the image plane corresponding to D* T ; and determining that the determined object is the relevant object responsive to a determination that the absolute distance between the nominal feature point coordinate O* 2t and the second feature point coordinate in the first captured image frame O 1T is less than the absolute distance between the second feature point coordinate in the second captured image frame O 2T and the second feature point coordinate in the first captured image frame O 1T .