1. An obstacle detection method, comprising: (a) disposing a camera at a vehicle, the camera having a field of view external of the vehicle; (b) acquiring a series of image frames captured by the camera whilst the vehicle is in motion, the image frames defining an image plane having a vertical aspect and a horizontal aspect; (c) detecting a first object present in the series of image frames by edge detection; (d) detecting a second object present in the series of image frames by edge detection; (e) for the first detected object, (i) selecting first and second feature points from the first detected object that are spaced apart in a first image frame, (ii) tracking the positions of the first and second feature points over at least a second image frame, and (iii) determining the movement of the first and second feature points when projected to the image plane; (f) for the second detected object, (i) selecting third and fourth feature points from the second detected object that are spaced apart in a first image frame, (ii) tracking the positions of the third and fourth feature points over at least a second image frame, and (iii) determining the movement of the third and fourth feature points when projected to the image plane; (g) comparing the movement of the first and second feature points when projected to the image plane to the movement of the third and fourth feature points when projected to the image plane; and (h) distinguishing between the first detected object being a three dimensional object and the second detected object being a two dimensional object by determining that the movement of the first feature point when projected to the image plane is dissimilar to that of the second feature point and by determining that the movement of the third feature point when projected to the image plane is similar to that of the fourth feature point.