Big Dig managers have complained for years that "linear heat detectors," which are supposed to trip alarms when temperatures rise drastically in the tunnels, have proven virtually useless in the Ted Williams Tunnel. That point was made clear in May, when the $1 million system failed to sound the alarm until after firefighters had extinguished an engine fire on a bus carrying the Seattle Mariners baseball team.

While Fire Commissioner Paul Christian has agreed the heat detectors have not worked well, he says firefighters need a different, costly item to fight tunnel blazes: fire engines that can shoot powerful streams of flame-suppressing foam. The reason, Christian said, is that firefighters take about three times longer to reach tunnel fires than above-ground fires, and, as a result, underground blazes tend to burn much more stubbornly.

Foam is much lighter than water and easier for firefighters to spray, shoots farther than water, causes less damage, and preserves evidence better for investigators, Christian said.

The fire department housed at the Massachusetts Port Authority's Logan Airport uses foam-spraying trucks, but the trucks are too large to enter the Central Artery's confined tunnel system, Christian said.

"We feel the notification side is adequate without the linear heats," Christian said. "What we need is to beef up the extinguishing side."

Fire Department and Big Dig officials have held meetings recently to reach a compromise. Late last month, Michael Lewis, the Big Dig's project director, established a task force to examine the fire engine purchase, which would mean spending an unplanned-for $2 million.

"Our key considerations are to protect the public, and protect the infrastructure, and to get the highway up to normal operating conditions as quickly as possible" after a fire has broken out, Lewis said. "We want the tunnel system to be state-of-the-art, but we have determined that the linear heat detectors aren't the best way to go."

In recent years, a series of deadly tunnel fires in Europe has sent engineers scurrying to figure out the best ways to minimize the effects of vehicle fires, where toxic smoke and confined heat can wreak havoc.

To minimize the risk of fire in the Big Dig's tunnels, speed limits will be 45 miles per hour, Lewis said, as opposed to the 55- and 65-mile-per-hour limits on other parts of the interstate highway system. Also, traffic will go in only one direction in each tube, reducing the risk of head-on collisions. And a system of height detectors and cameras will help highway operators to spot trucks that are too large or carrying hazardous materials before they reach the tunnels. No hazardous materials will be allowed in the subterranean roads.

The project used information gathered from tunnel-fire tests in an abandoned underground highway in West Virginia for its fire plan, which features several devices that together constitute a highly sophisticated fire and emergency detection system. Closed-circuit cameras will allow operators to watch the tunnel 24 hours a day, carbon monoxide detectors will measure air quality, and subsurface motion detectors will sound alarms if traffic suddenly stops.

In the event of a fire, preliminary plans call for the Fire Department to dispatch companies from any of seven fire stations located along the tunnel, as well as from Brighton, where fire engines could enter the Interstate 90 portion going east. Also, several emergency stations along the route will give tow trucks, State Police, and ambulances direct access to the tunnels.

The tunnels were built with standpipes capable of pumping thousands of gallons of either water or foam from street level. That means that if firetrucks cannot gain access to an incident scene because of traffic, firefighters can still deliver water or foam to quell a blaze. Meanwhile, massive fans will move smoke and hot air out of the tunnel and through ventilation stacks that now punctuate the city's skyline.

However, questions remain about how effective the tunnel alarm systems will be. In the Mariners' bus fire, the "loop" detectors that monitor traffic flow failed to sound the alarm in the highway's Operations Control Center, according to the official incident report. That happened even though traffic came to a halt as motorists abandoned their cars and ran for the exits.

Lewis says glitches in the loop detection system in the Ted Williams Tunnel have been fixed and that the system now works well. In addition, he said, the system installed in the highway portions opening this fall was installed differently and has already been successfully tested.

Christian said that his department has tested its radio communications in the new tunnels, and that system works, too.

In the bus fire, the incident report shows, highway operators learned of the blaze from a carbon monoxide alarm. A camera then focused in on the smoking bus, allowing the operator to record the incident soon after it had begun.

The incident proved a good test for the Big Dig's operators, who are working to complete a comprehensive emergency management plan for the 7 1/2 miles of tunnels, Lewis said. Without such a plan, the roadways will not be allowed to open.

The first major opening, the Turnpike extension to Logan, is scheduled Nov. 8. A month later, the northbound section of the Expressway is to open.

However, the foam trucks, once ordered, take approximately six months to arrive, and the Fire Department will not have the equipment it says it needs when the roads open.

But Lewis said he is quite certain that the new roads will not open unless the Fire Department is sure they are safe.

"I'm confident the two agencies will work through any problems so it's safe to open the facility," Lewis said. "We won't open otherwise."