Why traffic flow slows when drivers hit tunnels

NORFOLK

Traffic is moving at a decent clip until... the dreaded tunnel.

For no obvious reason, brake lights illuminate ahead, speeds slow, and bumper-to-bumper conditions reverberate back through what has become heavy traffic.

Tunnel tie-ups are a part of daily life in Hampton Roads. There's no denying that at peak times more people want to cross the region's four tunnels than the spans are designed to carry.

But are the actions of the folks behind the wheel inflaming the problem?

We hit the brakes because we're uncomfortable with dark, enclosed spaces.

We follow too closely.

We don't maintain consistent speeds.

We forget to gas up or fail to keep our cars in good condition.

Experts say changes in driver behavior could squeeze a few more cars through the tubes, making commutes a bit faster.

It won't erase the need for additional capacity, however.

A pair of billion-dollar tunnel expansion projects are under review by state highway officials. Last week, a group of private companies proposed doubling the size of the Hampton Roads Bridge-

Tunnel. Some of the same companies are close to an agreement to build a second Midtown Tunnel.

And a third tunnel crossing of the Hampton Roads harbor is still on the drawing board.

Meanwhile, can drivers do anything to help ease the pain?

The problems that limit tunnel flow can be categorized into things drivers can do something about and things that they can't.

Experts say there are psychological and perceptual reasons that many drivers hesitate at tunnels.

"Some people who are nervous about confined places and dark places respond to that nervousness by slowing down," said Mark Scerbo, a psychology professor at Old Dominion University.

It's a response to having less freedom of movement, he said.

Asad J. Khattak, an engineering professor who leads an ODU transportation program, said drivers perceive a narrowing of the lanes, even though the lane width remains the same.

"The fact that you have a wall next to you is a very limiting factor," he said.

There's also a reaction to the change in brightness.

"Our visual system will adapt to the overwhelming change in intensity of lighting, but not instantly," Scerbo said. It takes about 20 minutes for the eyes to fully adapt to dark conditions, he added.

"We reflexively hit the brake and slow down to give our eyes time to adjust," he said.

Those things are tough to change, but other driving behaviors that impede traffic flow can be controlled.

Following too closely and failing to drive at consistent speeds slow traffic because they cause sudden braking.

"Constantly adjusting and changing speeds causes ripples and shockwaves in traffic," Khattak said.

When a road is jammed with traffic, the driver of one car brakes, then cars behind it brake in succession to avoid crashing into the car ahead. A shockwave results.

Because of traffic-flow dynamics, the position of the clump of slower cars moves back through congestion and grows in size, said traffic engineer Dwight Farmer, who heads the Hampton Roads Transportation Planning Organization. That section of stop-and-go traffic moves back about one mile every 7-1/2 minutes, he said.

So if one nervous driver brakes at the mouth of the tunnel at rush hour, the shockwave of stop-and-go or slow-and-go conditions will hit a driver 15 minutes later two miles before the tunnel.

Farmer said the shockwave dissipates only when the demand dips below capacity.

"If everybody goes the same speed and distance and nobody uses the brakes, you can through-put more vehicles," Khattak said.

These tunnel-driving behaviors are so notorious that the proposal for the group interested in widening the HRBT suggests making the tunnel four lanes in each direction and the approaches only three lanes.

Scerbo noted, though, that regardless of how many lanes are added at the tunnels, many drivers will still brake.

Although some of these driver reactions can't be controlled, the physical environment they're reacting to can.

"We've tried to counter this whole perception that you're driving into a hole in the water... so you don't feel like you're going into a different environment than you're coming from," said Dwayne Cook, regional operations manager for the Virginia Department of Transportation.

VDOT has worked to try to maintain radio and cell phone signals in tunnels. Lighting levels are graduated with brighter entrances and exits, and entrance walls are painted a pastel peach.

Tunnel tie-ups, however, are most often blamed not on what drivers are doing, but on what they're driving.

Cook is fond of saying, "Don't drive drunk and don't drive junk," a lighthearted take on what he says is a serious matter.

Mechanical issues - including stalled vehicles, flat tires and empty gas tanks - account for 50 to 70 percent of traffic stoppages at local tunnels, Cook said. A good portion could be prevented, he said, with better vehicle maintenance.

Traffic is stopped an average of 15 minutes for emergency crews to change a tire or tow a vehicle that has stalled or run out of gas, according to VDOT statistics.

Every minute that traffic is blocked during a peak period results in four minutes of delay, according to the National Traffic Incident Management Coalition. So at rush hour, one incident can result in an hourlong backup.

Last year, an average of about four incidents a day each occurred at the HRBT, the Downtown Tunnel and the Monitor-Merrimac Memorial Bridge-Tunnel. At the Midtown Tunnel, the average was one stop a day.

That does not count stoppages for accidents, fires or turning around trucks that are too tall.

Running out of gas, a very preventable problem, happened, on average, once a day at the Downtown and the Monitor-Merrimac tunnels last year. It happened about every other day at the HRBT, and about twice a week at the Midtown.

"If people want to improve congestion at the tunnels," Cook said, "make sure you have gas, check your tires and make sure your vehicle is in good working order."

Debbie Messina, (757) 446-2588, debbie.messina@pilotonline.com