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There's a science to timing traffic lights

Tuesday - 6/18/2013, 3:36am  ET

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WASHINGTON - On your commute there's probably one traffic light that just drives you crazy. You're not alone, but there's a reason traffic signals work they way they do.

Traffic lights facilitate the flow of traffic, and an array of traffic signals working harmoniously regulates the flow of traffic across a broader region.

In densely populated areas, however, a network of signals can quickly become overloaded during peak travel times. That's when frustration sets in.

An unofficial WTOP poll conducted in late May asked local motorists to identify the intersections that cost them the most time. The map below illustrates the widespread nature of the grievances aired by these commuters.


View Traffic Lights Poll in a larger map

The process of coordinating a network of traffic lights can be complex and exhaustive. Detailed studies and weekly surveys account for drivers traveling roads of varying capacity across a broad area to various points at different times of the day. The balance between optimal and gridlock is often tenuous.

Wayne Wentz, chief of transportation, engineering and operations for Arlington County's Department of Environmental Services, says highway engineers often work on razor-thin margins when timing a series of traffic signals.

"You have to essentially time a whole corridor for the worst intersection in that corridor, two major arterials (that) both need a certain amount of time to serve them. All the other streets on each corridor then need to match that same cycle length if we're going to synchronize," Wentz says.

It's not just vehicles - planners also have to factor in pedestrian traffic patterns at nearby crosswalks.

Wentz says that long cycle lengths are pedestrian-unfriendly and can lead to jaywalking and other unsafe behaviors, which can result in bigger problems for road users.

Highway engineers classify the batch of vehicles that accelerate away from a green light as a "platoon."

"We try to do a time-space analysis," Wentz says. "If any particular signal lets a platoon of cars go and that platoon is going to travel at a certain speed our goal is to progress every platoon all the way through the length of roadway.

"We know the length of the average platoon of 15, 20, 30 cars and we know how far apart the signals are. If you take the distance divided by the speed, you know how long it should take that platoon to get from signal to signal to signal."

This simplified approach to traffic management relies on an idealized traffic flow. Wentz says it gets more complicated with added volume.

"The greens are supposed to be long enough for every platoon, but because we load some blocks up with (traffic from) side streets and driveways and because speeds aren't perfectly regulated, platoons start to break up. We can't perfectly predict the volumes."

Throughout the region, various jurisdictions are charged with the responsibility of traffic signal coordination and control. Although a series of lights may be coordinated along a stretch of roadway, they may not be synchronized across jurisdictional lines. Backups can therefore result in corridors that span county and city lines.

WTOP contacted a few of these local governments and inquired about the most troublesome intersections identified in the poll.

Most of the representatives insist that a mistimed signal is a rare occurrence.

In many cases, what seems like a poorly-timed intersection from one vantage point is functioning properly in a larger system. In other cases there is simply too much input.

The road network is then said to be over capacity, and the traffic signals are overwhelmed.

"Every intersection has a capacity. There are times when the demand volumes can exceed that capacity and that's when the level of service falls," Wentz says.

When traffic volumes increase, synchronization of signals on one road gives way to the optimization of the regional traffic flow on all of the roads. This, Wentz says, comes at a cost.

"If you have two main streets that are crossing each other, you've got to (regulate) both of those corridors."

In other words, what is good for the individual is not necessarily best for the whole.

Still, there will always be a running list of traffic signals that seem to defy the greater good.

"Sometimes there are faults in the system. Sometimes it's worth calling your jurisdiction and saying, 'Have you noticed there's a change?' But it can also be that there are just situations that people aren't aware of. It may be that the particular signal they're going to is optimized for the intersecting street," Wentz says.

The agencies that oversee traffic lights on major arteries do their best to flush out weaknesses that may arise from irregularities in the traffic pattern. Accommodating everyone's needs can often be challenging, if not impossible.

"Like anything you can think of, sometimes stuff breaks — no conspiracy, just the way it is," WTOP Traffic Reporter Bob Marbourg says.

"Even when the engineers try to optimize and maximize the flow of traffic, the solution will almost always be a compromise but hopefully without compromising safety."

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