1. A system to detect the transit of vehicles having license plates, the system comprising: at least one video camera to detect license plates capable of framing the license plates of said vehicles and of generating a corresponding video signal, and a first processing chain sensitive to said video signal to perform license-plate-recognition processing on said video signal, at least one video camera to detect said vehicles capable of framing a zone of transit of said vehicles having license plates and of generating a respective corresponding video signal, and a second processing chain sensitive to said respective video signal to perform on said respective video signal vehicle-tracking processing to detect the position and three-dimensional shape of said vehicles in transit in said zone, said first processing chain including a test module to detect the presence of a vehicle in transit by performing a test as to whether an image of a license plate persists on a series of images of said video signal, and a plate-tracking module which is activated by said test module when the license plate persists on the series of images, a supervisor module that aggregates the results of said first processing chain and said second processing chain to generate information records, each record of said records identifying the transit modality in said zone of a vehicle identified by a given license plate tracked and recognized by said first processing chain.