Pictures of the Future      Fall 2005

Improving Tunnel Vision

On March 24, 1999, a truck burst into flames in the Montblanc Tunnel and 39 people died in one of Europe’s worst tunnel disasters. It took firemen 53 hours to bring the fire under control. Because of the smoke and 1,200 °C temperatures, they were unable to reach the burning truck.

This and subsequent catastrophes generated a vigorous debate and led to changes in regulations. "Today, as a result, a parallel safety tunnel and video detectors have become standard features," says Karl Rohrer from the Construction and Property Development Department of the Swiss canton of Obwalden. Rohrer was responsible for the safety features of the 2.1-km Giswil Tunnel, which connects Lucerne and Bern and is today regarded as the safest and most modern tunnel in Europe. On entry, motorists immediately notice the excellent lighting, the niches equipped with emergency telephones and fire extinguishers, and the brightly illuminated escape tunnels. Traffic is observed around the clock by 23 video cameras. In addition, sensors measure visibility impairment, wind direction and temperature. This data is in turn used to continuously adust ventilators, lighting and traffic guidance.

Rohrer considers the video monitoring system, which was installed by Siemens in 2004, as the key visual aid. In case of an emergency, the systems trigger a chain reaction of alarms and safety measures. "For example, the system can detect a fire at an early stage by analyzing the development of smoke," says project leader Michael Ludwig from Siemens Building Technologies (SBT) Switzerland. "It can also identify stalled cars and congestion."

The system uses evaluation algorithms to calculate how many vehicles are in the tunnel and their speeds. Using gray-scale video analysis technology, any deviation from standard reference images is closely observed. "If there’s a long-term change, for example if a vehicle halts because of a flat tire, the system sounds an alarm. We can then halt the traffic in front of the tunnel and thus prevent congestion inside," says Ludwig. The biggest advantage of video detection is the automatic alarm system. In case of an emergency, these intelligent "eyes" reliably save the relevant data and transmit the images in real time to police headquarters. Because the video cameras never tire, they save a lot of work for their human teammates.

At the heart of the system is a master computer, which is linked with other computers in the network via a Siemens Profibus data transfer system. The fire alarms, ventilation system, alarm signals and traffic monitoring are thus all linked. If thick smoke is detected, a message is immediately sent to the various safety systems. These, in turn, switch the traffic lights at the tunnel entrances to red, make the lighting brighter and activate the smoke outlet.

A test fire demonstrates the effectiveness of the system. On the video monitor, the fire is just a bright light. Clouds of smoke rise and the image grows dimmer until the screen turns gray. But suddenly there’s a whirl of air and the cloud disappears. The system has sucked the smoke out, thus reducing the danger of smoke inhalation and making it easier for drivers to find escape routes. The smoke outlet system includes vents spaced at 75-m intervals along the tunnel’s ceiling. By automatically opening at the right places in case of a fire, these events enable the ventilators to draw off 150 m&3sup; of air per second through a discharge chimney.

The escape tunnel also has ventilators that are operated in order to increase the difference in air pressure between the two tunnels. Engineers installed this mechanism to prevent smoke from entering the escape tunnel when its doors are open.

All of these safety measures are managed through a higher-level control system, which has access to the controls of the subsystems. The ventilation, lighting and tunnel closing systems are automatic, but the control center at police headquarters can intervene at any time. "Without a video analysis system, the other systems would require a much longer reaction time," says Ludwig. "It gives us information at an early stage so that we can intervene in the right way to avert a catastrophe." Karl Rohrer is also satisfied with the system. "So far, we’ve had practically no false alarms," he says. Even more to the point, there have been no major accidents.
                                                                                                         Evdoxia Tsakiridou