May 1998

The Big Dig

To bury Boston's busiest highway underground, engineers are simulating traffic flows on their computers (to give the highway a brain) and starting huge fires in West Virginia (to give it a fire control system).

By Simson L. Garfinkel

When president dwight d. Eisenhower signed the Federal Highway System legislation in 1956, he couldn't possibly have envisioned the behemoth construction project that is now being undertaken in Boston to complete the system. Government officials call it the Central Artery/Tunnel (CA/T) Project; locals just say "the Big Dig." By the time it's finished in 2004, this roadway will boast a segment eight lanes wide, 3.5 miles long, completely buried beneath the bustling financial district of one of the nation's oldest cities. The new tunnel will replace Boston's much-maligned Central Artery-a dilapidated steel viaduct that cuts between downtown high-rises-with a stretch of the world's largest underground highway. An underwater tunnel (completed in 1995) will feed traffic from the airport into the artery, all for the unprecedented cost of more than $10 billion.

Although burying the highway promises to leave an improved environment for Boston's surface dwellers-cleaner, quieter, more open-it raises the stakes for subterranean travelers. Traffic jams and flat tires, merely annoying above ground, can turn deadly below if motorists are trapped in a haze of toxic exhaust fumes. Add to the mix a car fire or an oil tanker explosion and the situation could become dire. So Big Dig engineers are pioneering new technologies in construction, traffic management, and fire control, all designed to keep life flowing smoothly and safely through the artery.

The Brains behind the Operation

beyond laying steel and pouring concrete, Big Dig crews are deploying hundreds of closed-circuit television cameras, infrared sensors, and variable message signs throughout the system, wiring it together with a computer system that can withstand a terrorist attack, and building a command center so filled with screens, keyboards, and projection devices that it would make Darth Vader green with envy. It's all part of the Central Artery/Tunnel "Smart Highway," or Intelligent Transportation System.

Working in the "Star Wars" control center, the CA/T's half-dozen human operators will strive to maximize traffic flow and minimize motorists' exposure to carbon monoxide. The tools at their disposal will include traffic lights, speed limit signs, lane closure signals, AM and FM radio transmitters, ventilation equipment, even sewage pumps.

The CA/T's computers will constantly monitor the flow of traffic through the system. If there is a sudden interruption-say the traffic in a lane drops from 60 to 5 mph-the computer will automatically swing a camera to point at the area in question. The computer can calculate the severity of the incident, designate an appropriate human operator to handle it (based on his or her training and current assignments), and make the video image appear at the operator's console. Then the computer will recommend a strategy for handling the situation, but leave the final decision to the human, who can change lights, adjust ventilation equipment, or send messages to drivers, all to prevent a minor fender-bender from becoming a major catastrophe.

But what should an operator actually do in an emergency? Close lanes? Slow traffic? Divert traffic? And how long should lanes stay closed? To answer these questions, the Massachusetts Highway Department contracted with MIT's Intelligent Transportation Systems group (http://its.mit.edu/) to build an advanced computer simulation that models up to 10,000 vehicles moving through ramps and tunnels.

"We simulate the drivers' decisions such as acceleration, deceleration, lane-changing, merging, and yielding," says professor Moshe Ben-Akiva, who directs the MIT group. "We can simulate incidents by blocking lanes for a certain duration. We can simulate changes in visibility conditions." The system can even determine the effect of closing exits or adding new ones.

Along with the traffic simulator, the MIT group has built a second simulator that models the CA/T's human operators and traffic management system. This lets the researchers see the effect that different traffic management strategies will have on the ebb and flow of traffic inside the tunnel. When there is an accident inside the tunnel, for example, the portal lights on the freeway immediately turn from green to red to prevent more cars from entering. Using the simulator, the researchers calculated how long the operators should wait after the accident is cleared before the portal lights are turned from red back to green. "The original plans to change the portal lights to green immediately was not a good idea," says Ben-Akiva. "You should delay the change until you let the traffic inside clear out. Otherwise, you generate shock waves of traffic inside the tunnel."