A system (1) for detecting the speed of vehicles, particularly for sections of roads and highways, comprising a plurality of peripheral units, each of which comprises a unit (2) for detecting the class and speed of a vehicle, and at least one video camera (3) for acquiring the image of the license plate of the vehicle, the peripheral units being connected to a central processing unit (5), the peripheral units being further provided with a peripheral processing unit (11) that is adapted to identify the license plate acquired by the video camera, the central processing unit (5) determining the average speed of the vehicle on the basis of two distinct detections performed by two successive peripheral units.