Dave Hall, of Point Breeze, takes the Fort Pitt Bridge and Tunnel and the Parkway West to his Robinson office.

As he approached the entrance to the outbound tunnel June 13, the emergency traffic lights on the bridge turned red. The driver of a car ahead of him slammed on the brakes, triggering a chain-reaction accident. Although he stopped in time, Hall was rear-ended, causing almost $1,000 in damage to his car.

The vehicles were drivable. Within a minute, a PennDOT tunnel worker waved everybody through to the south portal, where they pulled to the side and exchanged insurance information.

"There didn't seem to be a problem inside the tunnel that warranted the red light being activated," Hall said. "Thank God, no one was injured or killed. A co-worker said tunnel personnel will sometimes activate the light to cross from one side to the other."

Sorry to hear about the accident. It's a story that happens too often on the Fort Pitt Bridge, a transportation linchpin but notorious bottleneck for 140,000 vehicles a day.

So maybe a bit of enlightenment about the purpose and operation of the emergency signal system will prevent the next fender bender. The source of the info is PennDOT District 11 Tunnel Manager Tom Diddle, and, of course, the PG's Roads Scholar.

While traffic lights are a rarity on the nation's 45,000 miles of interstate highways, it probably comes as no surprise that Pittsburgh is an exception. The signal system is designed to stop trucks exceeding 14 feet high from entering and getting stuck in the tunnel because of height clearance limits.

Tunnel workers are able to override the system manually to stop traffic in other situations, such as an accident, breakdown or dangerous buildup of noxious exhaust fumes.

They don't stop cars to cross from one tunnel to another. They use staircases behind the tunnel facades to get to upstairs rooms and crosswalks, where ventilation equipment and controls are housed.

What happened in Hall's case, Diddle speculated, was that an overheight truck came down Green Tree hill, triggering inbound traffic lights near the tunnel entrance and sounding an alarm to alert PennDOT workers to the situation.

When the trucker stopped, the workers manually activated the signals on the outbound level of the Fort Pitt Bridge, thereby closing the outbound tunnel. After they crossed the rig to the opposite side of the Parkway West and sent the driver outbound, the workers reopened the tunnel.

The move takes only a few minutes, but when it happens, "People don't see an apparent problem and wonder why they were stopped," Diddle said.

One or both groups of emergency lights on both sides of the tunnel typically are activated several times a day on weekdays and up to a dozen times on a really bad day.

There's more you should know:

Because outbound traffic is relegated to the basement of the dual-level bridge, and because the deck is four lanes wide, structural features prohibit the lights and directional signs from being placed where they can always be seen easily. A "Be Prepared to Stop" sign is posted, as if anyone local needs to be told.

There are four "signal heads" -- traffic lights -- inbound at the bottom of Green Tree hill, and four outbound on the Fort Pitt Bridge.

The lights now sequence from green to yellow to red, the same as conventional traffic signals, an improvement over tunnel lights that snapped from yellow to red for the previous 25 years.

Once I explained everything, Hall was satisfied. He, too, thought a refresher course on the tunnel emergency traffic signal system would be an easy subject for a transportation writer in a hurry to get out of town.

By the way, next month will mark two years since PennDOT finished the 11-year, $200 million renovation of the Fort Pitt Bridge and Tunnel. Time flies.

Sensors make sense. On July 1, another tractor-trailer flipped over on Interstate 79 south of Neville Island, despite the fact that PennDOT has installed numerous warning signs about the dangerous S-curves.

Because speeding is usually to blame, Art Peternel, of Franklin Park, has an idea worthy of PennDOT's consideration.

"Install sets of sensors similar to those that monitor overheight vehicles at the Fort Pitt Tunnel, but ones that can monitor speed," he e-mailed.

Peternel suggested using cameras to capture license plates of offending truck drivers and then sending a warning about driving too fast for conditions. "Take further action against repeat offenders," he said.

Safety first. Two motorcycle accidents, one fatal, in two months drew public attention to the sharply curved ramp leading from Route 28 south to Veterans Memorial Bridge, a high-accident site since it opened in the late 1980s.

A recent Pittsburgh Post-Gazette report showed virtually nobody obeyed the 30 mph speed limit; one car was clocked at 64 mph.

PennDOT has responded by posting more signs at the tricky curve. It is considering raising the 32-inch-tall concrete barrier wall by 10 inches or erecting a fence to try to keep vehicles and bikers from flying over the edge.

Mr. Know-it-all has another idea: Install pavement "rumble strips" on the bridge ramp. I hate those things. But they always get my attention and they always prompt me to slow down.

PennDOT also should use the adhesive-backed strips, available in "reflective yellow," on the spaghetti of troublesome Fort Pitt Bridge and Fort Duquesne Bridge ramps around Point State Park.

Elsewhere. The decision was not made without prolonged controversy, but Seattle has entered into agreement with a consortium of firms to build a $1.9 billion, 14-mile monorail by December 2010. Seattle has operated a mile-long monorail to its Space Needle for decades.

Believe it. A claims study conducted by The Progressive Group of Insurance Cos., the country's largest motorcycle insurer, showed the Suzuki GSX-R Series is not only the motorcycle most likely to be stolen but also the one most likely to be wrecked. The least likely to be wrecked? Yahama Virago Series.

Plate du jour. David Fink, a PG sports copy editor, spotted the Pennsylvania personalized license plate SPUTNIK on a car in Mt. Lebanon. Probably has satellite radio.