1. A system for measuring the speed of a vehicle having a feature of known size, comprising: one or more cameras for capturing images of vehicles on a road; timing means for timing the capture of the images; a processor arranged to operate in at least a calibration mode and a monitoring mode, wherein: (i) in the calibration mode the processor: identifies in the captured images the feature of a vehicle; calculates the size in pixels of the feature of the vehicle in two or more of the captured images; receives as an input the vehicle's geodesic distance along the road at the times of capture of the captured images; and generates a look-up table or function correlating the calculated size of the feature to the vehicle's geodesic distance along the road; and (ii) in the monitoring mode the processor: identifies in the captured images the feature of the vehicle; calculates the size in pixels of the feature of the vehicle in two or more of the captured images; determines the vehicle's geodesic distance along the road in each image by using the calculated sizes as an input to the generated look-up table or function; and determines the speed of the vehicle based on the determination of the vehicle's geodesic distance along the road in each image and the times at which each image was captured.