1. A computer-implemented method for generating a classifier for detecting objects in a digital image, the method comprising: providing, for each object in a set of objects in one or more digital images, a description of a roadway or a street in which or near which each object appears, the description including a corresponding road or street width estimate; generating, by one or more processors, a set of composite feature vectors by combining one or more selected feature vectors with the corresponding roadway or street description including the width estimate, each composite feature vector including attributes of a given one of the set of objects; and generating, by the one or more processors, an object classifier that filters out false positives using the composite feature vectors.