1. A vehicle detection system for detecting the presence of at least part of a vehicle in image data, the system comprising: an interface configured to receive image data; an identifier module configured to identify a plurality of linear regions in an image represented by the image data; a comparator configured to compare at least one of the number, cumulative size, and density of the linear regions with a respective threshold value; and an output configured to issue a signal indicating the detection of a vehicle based on the results of the comparison.