A traffic surveillance system is often installed when the level of traffic is very dense in a tunnel. Usually, a video-surveillance system is used, supplemented sometimes with counting devices. A video-surveillance installation provides the operator with important information in order to make decisions about controlling the traffic conditions in the tunnel in real time. In case of degraded or emergency operation, it allows the concerned incident zone to be viewed so that required actions may be rapidly evaluated.
Video-surveillance is thus a very valuable tool for the operator because it enables continuous incident monitoring inside the tunnel and rapid reactions when necessary.
However, in order to make full use of a video-surveillance installation, it is essential to maintain a human presence, if possible continuously, at the control-command centre.
Video-surveillance is generally quite simple in its design. Cameras placed at regular intervals in the tunnel provide a complete coverage of the tunnel and its surroundings. The images are then grouped and transmitted to the control-command centre of the tunnel by networks that may or may not be dedicated to this function. The images are then received and viewed on the displays (Fig. 1).
CCTV systems can also be relayed to automatic incident detection systems (to detect smoke, fire, accidents, traffic jams or intrusions for example). AID systems are especially useful for long tunnels, where it is difficult for the operator to observe the entire tunnel even with a wall of screens.