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District gridlock is the Traffic Signal System’s eternal enemy.
As anyone who was driving in the District on the morning of Sept. 11 will attest, the traffic system was among the city services hardest hit by the unprecedented events. The early closure of federal and local government buildings overloaded the city’s streets with a massive exodus of commuters and led to scenes such as the one glimpsed on the corner of Connecticut Avenue and K Street NW, where a lone man stood in the middle of the intersection and attempted to direct traffic, waving cars through with one hand as he held his briefcase and jacket in the other.
“When you’re bringing people in over a four-hour period, it’s very hard to disperse them over a half-hour,” says Bill McGuirk, the chief of the District’s Traffic Signal System Division. “As soon as we got word that the federal government had let off for the day, we went ahead and put in our p.m.-rush-hour timing plan.”
The challenges didn’t end that day. Numerous streets around the White House, the Capitol, and other buildings in federal D.C. have been closed at varying times for security reasons, and each closing has sent a ripple of disruption through the city’s traffic system.
McGuirk has worked to maintain the District’s traffic grid since 1973, so his claim to preparednes for the bevy of recent adjustments that he’s had to make possesses an air of authority. “When you start closing streets, you start impacting the city’s transportation infrastructure, and adjustments have to be made,” McGuirk says. “Sometimes it’s a minor adjustment; other times it’s adjustments that impact an entire corridor.”
The 1995 closure of the stretch of Pennsylvania Avenue in front of the White House offers an example of how such ripples of disruption are created. When the street was blocked off, the city lost eight moving lanes of traffic. At full capacity, McGuirk estimates, these lanes carried up to 1,000 vehicles in each lane per hour. All of those vehicles had to be reabsorbed elsewhere in the city’s traffic network.
McGuirk and his staff control the city’s traffic flow via 1,600 4-foot-tall gray boxes located on street corners throughout the city. These control devices often escape notice because of their secondary role as a space for protest placards, but they are the nerve endings of the city’s traffic systemcommunicating once every second with a command hub in the Reeves Center on 14th and U Streets NW.
“It’s basically a PC on the corner,” says McGuirk of the curbside “controllers.” Each unit costs about $10,000, and the city spends $2,000 per unit on electricity and maintenance each year. “Every intersection in the city has a specific operation, and each of our signals is interconnected,” McGuirk says.
The control room at the Reeves Center where the city’s main traffic computer is housed would make HAL from 2001: A Space Odyssey proud. From this space, the system’s central computer implements and maintains intricate timing plans for each traffic signal.
On one side of the room is a wall of flashing red lights, denoting communication via cable with the sidewalk controllers. In the center of a bank of computers is the system’s compact and slick-looking brain.
Operational Support Manager Mike Travers lifts up the top of the computer and affectionately describes the components in the surprisingly empty machine. “He’s 2 years old, so he’s ancient,” says Travers. “At the time, he was top-of-the-line. He’s a dual processor450 megahertz. He has about 1 gigabyte of RAM. He has a fairly small hard driveabout a 2-gigabyte hard drive. The database servers have about 20-gigabyte hard drives.”
The computer’s work is fairly complex. Each signal has an individual timing plan based on traffic volume, accident history, and other factors. They are also synchronized to interact smoothly with surrounding intersections and street corridors, and their timing is dictated by schemes that McGuirk’s engineering staff develop and constantly update.
Despite its high-tech contraptions, the District’s traffic system has one element that may hearten technophobes: All of its computer wizardry depends on fading green graph papers dating back to the ’60s and covered with pencil scribbles. These papers fill eight blue binders that McGuirk calls his “time/space books,” and he deems them essential to the calculation and charting of traffic flow in the District.
“If I lost these books,” says McGuirk, “I’d probably retire.”
The time/space binders chart the progress of traffic along street corridors in north-south and east-west directions, with the green graph papers presenting the streets and each of their intersections to scale. Crosswalks have individual Manila folders that also predate the computers, and even the District’s first stoplightsinstalled on Pennsylvania Avenue in the ’20sare represented in McGuirk’s file cabinets.
The goal of all the math in the binders is to get the lights exactly right. On most one-way streets in the District, if traffic is flowing unimpeded at 25 mph, cars will encounter a steady progression of green lightswhat McGuirk calls the “green band.”
Preserving such timed cycles is a perpetual struggle, however. For every addition of a left-turn signal, a new light, or a closed street, the cycle must be tweaked. The evidence of these changes can be seen in the time/space books, with eraser smudges and Scotch tape marking parts of the charts.
The system also takes pedestrians into account, calculating clearance times at crosswalks at close to 4 feet per second. These clearances can also be adjusted lower, to a pace of 3.6 feet per second, say, if an area is populated by a number of elderly citizens or children.
Amid the tweaking, however, McGuirk says he strives for consistency in the cycles, so that drivers recognize an orderly plan underpinning the system. “My problem is, 100 percent displaying this [system] on the street is almost impossible because of all the minor equipment problems and communications failures,” he says.
The ’60s-era traffic-timing plans are designed to implement three basic traffic patternsmorning rush hour, evening rush hour, and “all other times.” Such schemes are applied systemwide, so that when news of the Sept. 11 attacks and closures came to McGuirk, he quickly reset the program to evening-rush-hour modea move that he says was “the best and quickest thing that we could do at that time.”
The system’s software is equipped to handle more than three plans, however, and McGuirk says that an upgrade is in the cards. He hopes to implement more specialized varieties of timing cycles, including schemes for lunch hour, late night, and Saturdays.
When things go wrong, complaints come rushing in. On the morning when I meet with him, McGuirk has already received two calls from commuters about signal problems by 9:30 a.m.
Such calls keep the traffic system in a perpetual state of fine-tuning, and McGuirk says that his staff depends on the feedback. “Commuters are typically like sheep,” he says. “They follow the same route, and they become very, very familiar with it. When something becomes a little bit out of whack, they notice, and they tell you.”
System glitches are often caused by a drift in the synchronization between signals in a particular street corridor. Such drift can be caused by anything from cable failures to software bugs. With 64,000 traffic-signal light bulbs in the District, such failures can even boil down to something much simpler.
“You can get one little $2 piece to fail, and that could have ramifications that mean thousands of hours of delays,” says McGuirk. “Most of these are pretty easily solvable. It’s rarely something catastrophic.”
One localized catastrophe, however, is the loss of a controller. Although the ubiquitous street-corner boxes contain expensive electronics, they also are designed to break away if a car slams into them. Washington loses about 50 boxes a year to curb-jumpers, with the most frequent losses during the holidays.
A mass exodus from the District puts major strains on the system, but a nightmarish gridlock could also occur if traffic lights were knocked out during an evacuation of downtown Washington. Disabling all of the city’s traffic lights individually would be exceedingly difficult. But traffic-system employees say that as long as the time/space books were saved from headquarters in a disaster, all hope wouldn’t be lost.
“We can survive and be completely independent from the main building,” says Travers. “The controllers have about 90 percent of what’s in here. So if this room blew upexcept for the fact that we’d lose the ability to change thingsthe controllers would run for quite a while with their own knowledge.” CP