1. A method for automatic spatial calibration of a network of cameras along a road, the method comprising: processing a frame that is obtained from each camera of the network to automatically identify an image of a pattern of road markings that have a known spatial relationship to one another; using the identified images to calculate a position of each camera relative to the pattern of road markings that is imaged by that camera; applying geographical information to calculate an absolute position of a field of view of each camera; and applying a global optimization to adjust the absolute position of the field of view of each camera of the camera network relative to the absolute positions of the fields of view of other cameras of the camera network.