Properly timing signals is one of the most important things engineers can do to improve traffic flow. Adjusting the red and green is quick and cheap, compared with adding lanes, building more roads or creating new transit lines — and it’s less disruptive to the landscape than any of those things.
But there’s one miracle the signal engineers cannot perform: They can’t manufacture time. They can only redistribute it to meet differing demands.
That’s what the Virginia Department of Transportation does every few years along various commuting corridors. Drivers are sensitive to signal timing, and they’re not always satisfied with the results.
Dear Dr. Gridlock:
Most mornings, the northbound traffic on Reston Parkway is stop and go from before the Lawyers Road and Reston Parkway intersection all the way to the toll road. It seems when one light turns green, the next light turns red. And the opposite is true in the evening.
— Jeff Edelheit, Oak Hill
That stretch of Reston Parkway, on the south side of the toll road, is about two miles long. On the sides are houses, garden apartments, shopping plazas and office parks. It’s busy at rush hours, but it’s not always busy in the same way.
Nhan Vu, the VDOT signals operation manager in Northern Virginia, said the department is adjusting the signal timings along Reston Parkway, which should be done by the end of June.
The goal is to provide for the smooth progress of the heavy traffic to the toll road in the morning rush, and then from the toll road during the afternoon rush. A corridor is reevaluated every few years because traffic volumes change, and develop-ment can alter traffic patterns.
Vu refers to the review as “signal optimization.” He said it’s a bit different from syncing a set of lights in a city, where people may drive along routes with closely spaced intersections in a grid pattern. A suburban route may have very different spacing between intersections, and the intersection designs may vary.
Besides setting the overall pattern to move traffic to and from the toll road, signals can respond to the immediate demand at a particular intersection, thanks to sensors. During an intersection’s light cycle of 200 seconds, the through traffic on the parkway may be in line to get 105 of those seconds, with the rest distributed to cross traffic, or left-turning traffic from the parkway.
But maybe during one of those cycles, nobody wants to make a left turn, or no traffic is waiting at the cross streets. When sensors detect that, extra seconds can be transferred to the parkway’s main lanes and those drivers get an early green.
But there’s always that next intersection ahead. Some of the drivers who got the early green may get through it. Some, perhaps farther back in the pack, may have to stop for a red light. Others who are going to turn left or right at that second intersection may benefit because they get to the turn lane a little sooner than they would have.
The drivers on the opposite side of the parkway, the ones going against the rush-hour flow, also may have benefited from that early green at the first intersection.
Among the other things that can throw off even a well-timed light sequence: a pedestrian hitting the “push” button to cross an intersection.
Does all this explain Edelheit’s experience? He’s skeptical. From his point of view — the driver’s seat — the trip has been a mess since last month.
Edelheit suspects that the problem in the morning is with the lights at Sunrise Valley Drive and South Lakes Drive, two big intersections just south of the toll road, “and then everything backs up from there.”
Vu said the signal updates are based on traffic flow data and observation, but VDOT monitors the results and can make further adjustments if needed.
Because everybody cares about traffic-signal timing, a committee of the regional Transportation Planning Board prepared a report on the status of optimization programs. Reading the report, you get a sense that traffic planners feel a bit battered by the driving public: “ ‘Optimized’ . . . does not mean ‘without delay.’ The motorist may still experience delays even after signal or corridor optimization.
Issues regionwide match up with Vu’s description of the parkway corridor: high volumes, turning traffic, cross traffic, pedestrians and motorists traveling in the opposite direction from the dominant flow of traffic.
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, 1150 15th St. NW, Washington, D.C. 20071, or e-mail