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At busy intersections, timing is everything

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Properly timing signals is one of the most important things engineers can do to improve traffic flow. Adjusting the red and green is relatively quick and cheap, compared with adding lanes, building new 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 distribute it to meet differing demands.

That’s what the Virginia Department of Transportation does every few years along the various corridors used by commuters. Drivers are sensitive to signal timing, and they are not always satisfied with the results.

Dear Dr. Gridlock:
I live in a development just south of the intersection of Lawyers Road and Reston Parkway. About a month ago or so, something happened to the timing of traffic lights on Reston Parkway between the Dulles Toll Road and Lawyers Road. 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 after shortly turns red. And the opposite is true in the evening.
Jeff Edelheit, Oak Hill

Nhan Vu, the signals operation manager for the Virginia Department of Transportation in Northern Virginia, said the department is adjusting the signal timings along Reston Parkway. That program should be done by the end of June.

The goal is to provide for the smooth progress of the heavy traffic northbound to the toll road during the morning rush, and then for the heavy southbound traffic during the afternoon rush. A corridor like this gets reevaluated periodically because traffic volumes change over the years, and land development can change the 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 traffic 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 there’s no traffic 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 because they’re 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 earlier 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.

The only constant is that a signal engineer isn’t going to please all the people all the time. Among the other things that can throw off even a well-timed light sequence: A pedestrian hits a push button to get the crossing signal at an intersection.

Vu said the signal updates are based on traffic flow data and observation, but VDOT also monitors the results and can make further adjustments if needed.

Robert Thomson is The Washington Post’s “Dr. Gridlock.” He answers travelers’ questions, listens to their complaints and shares their pain on the roads, trains and buses in the Washington region.



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