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 wherein applying the global optimization comprises applying a rotation, translation, size adjustment, or shape modification to an absolute position of a field of view of a camera of the camera network, and wherein the rotation, translation or size adjustment is configured to align a projection into a horizontal plane of a marking image in the frame that is acquired by one of the cameras with a projection into the horizontal plane of a marking image in the frame that is acquired by a camera of said other cameras.