1. A method for detecting a signal structure from a moving vehicle comprising: capturing an image from a camera mounted on the moving vehicle; restricting a search space by predefining candidate regions in the image based on locations of signal structures along railway tracks stored in a memory; extracting a set of features of the image within each candidate region; and detecting the signal structure; wherein restricting the search space by predefining candidate regions in the image comprises mapping a set of measurement in world coordinates to a set of measurement in image coordinates based on a projection matrix.