Rush-hour traffic backs up near the Vienna Metro station, even when the shoulder lane, at left in photo, is open to through travelers. (Nikki Kahn/The Washington Post)
Columnist

Since the Active Traffic Management system was switched on in September, some Interstate 66 drivers have complained that it’s either too active or not active enough.

They say it’s too active when the overhead signs tell them to do things they think are unreasonable, like slow down. They see the system as too passive when the red “X” over the shoulder doesn’t switch to a green arrow when they could sure use another through lane.

Learning how to make the best of these new travel management systems is important to everyone — the commuters and the people who operate them — because the future of surface travel is going to be more about computers, cameras, sensors and signs than about asphalt for new lanes.

So I went to the McConnell Public Safety and Transportation Operations Center in Fairfax County and spoke with the people at the Virginia Transportation Department who manage the new information system.

I asked some of the questions drivers have been asking about what all the highway signs are supposed to do for travelers.

Ramp metering. At some I-66 entrances, drivers get stopped for several seconds by a red light just when they’re ready to accelerate onto the interstate. The signals have been around for a long time, but Active Traffic Management made them smarter.

Before, there were set times, based on traffic congestion history, when the meters operated. The new system puts the meters in play any time traffic builds.

“We never had this type of control,” said Hari K. Sripathi, the department’s regional operations director.

The goal is to spread the traffic out by slowing the pace at which vehicles join the through lanes. But “it’s not so much how many get on as how they get on,” Sripathi said. The few seconds of red breaks up the platoon of cars coming down the ramp. If each car pauses for just a few seconds, it means the drivers already in the through lanes won’t encounter a solid line of merging traffic, which could lead them to hit the brakes or change lanes, slowing down everyone.

Open shoulders. This is where the humans come in.Ramp metering and some other aspects of the traffic management system involve computers reacting to sensors and sending out instructions to equipment.

Switching the shoulder control from a red X to a green arrow is an operator’s decision, done with an assist from the computers and sensors. The tip-off comes when the sensors show a sustained drop in the average travel speed below 45 mph.

This can create a difference in perception between the operators and the travelers. The operators are looking at the average speed across multiple lanes in an entire segment of highway. Drivers are glancing down at their speedometers.

Even with slowing traffic, the operator still needs to confirm that the shoulder is clear of obstacles before opening it.

“We don’t want the system to automatically turn on the green arrow and create a safety hazard,” Sripathi said.

Shutting the shoulder again involves another judgment call. The operators look at the traffic volume as well as the speed. And they don’t want to open and close the shoulder every few minutes, which could confuse drivers.

Through lanes. The sensor system can spot trouble ahead in a lane — maybe a crash — and suggest that nearby overhead signs switch to a red X, closing the lane for traffic, while overhead signs farther back will display an angled arrow, indicating that drivers in that lane need to move over. But the computer’s solution needs to be confirmed by the operator.

Variable speed signs. When the electronic signs over lanes drop to, say, 45 mph, several things may be happening, and this is largely an automated thing. The system is trying to balance speeds across all the lanes for safety. “Smooth flow is the number one goal,” Sripathi said.

But the system also may want to slow drivers as they approach the back of a serious knot of stop-and-go traffic.

This part of the system may be beyond the control of the controllers. Drivers — well, most drivers — will respect the red over a lane, but they will go as fast as the surrounding traffic allows, until it doesn’t.

I’ll have more about the new system in an upcoming column. Meanwhile, write in and share your experiences.

Dr. Gridlock also appears Thursday in Local Living. Comments and questions are welcome and may be used in a column, along with the writer’s name and home community. Write Dr. Gridlock at The Washington Post, 1301 K St. NW, Washington, D.C. 20071, or ­e-mail drgridlock@washpost.com.