Why Syncing Traffic Lights May Not Reduce Congestion

By: Dave Roos  | 
traffic lights on New York street Broadway
This view of New York City's Broadway shows some heavy traffic and a variety of stop light colors. Does syncing lights mean that traffic will always keep flowing? Toshi Sasaki/Getty Images

In 2013, Los Angeles completed a 30-year, $400 million project to synchronize 4,500 traffic lights across the city's 469 square miles (1,215 square kilometers). By deploying a vast network of magnetic sensors installed under roadways plus hundreds of cameras, the city could now capture real-time traffic data using a centralized computer system to synchronize green lights in order to reduce LA's notorious congestion and rush-hour gridlock.

Soon after the Los Angeles system was completed, city engineers cheerfully reported that the average speed of traffic was up 16 percent, and that the time it took to drive 5 miles (8 kilometers) on LA city streets was down from 20 minutes to 17.2 minutes. And yet, in 2018 LA still held the No. 1 spot for most traffic-congested city in the U.S.


So why didn't syncing the lights make more of a dent in the congestion problem? Tim Lomax is a research fellow with the Texas A&M Transportation Institute and a 35-year veteran of traffic analysis. He says that adaptive traffic signals are absolutely a good thing, but they are not the quick-fix panacea that frustrated commuters believe them to be.

Synced traffic signals work best when there is a clear and predictable flow of traffic in one direction, explains Lomax. Think of the traditional commuter scenario in which traffic flows primarily from the suburbs into the city center in the morning, and reverses course in the evening.

"If you tell me that's the pattern of traffic flow, I can design a pretty well-functioning traffic signal system," says Lomax, because it's clear which side of the road should get longer green lights, and those lights can be timed with the flow of traffic to keep cars moving. The problem is that traffic patterns in the biggest U.S. cities aren't so cut and dried.

"If you look at the economic landscape now, there are jobs and populations spread all over most metropolitan areas, and the highest volume of commuting could be from one suburb to another suburb," says Lomax. "All of your critical bottleneck intersections have heavy traffic coming at them in both directions."


How Traffic Signal Syncing Works

California's City of Irvine explains the process like this: The city's traffic control center calculates the arrival time for a group of cars at each intersection, assuming the cars are traveling at a certain speed and then times the traffic signal to turn green just as this group hits the intersection. More "green time" is given to a main street with greater traffic volume than a side street with less volume.

Synchronized traffic lights don't mean that a driver will encounter an unending number of green lights as she drives down a major road. What it means is that all the signals on a main road are set to run the same cycle length (the time the signal goes from green to yellow to red to green) after the side street has been serviced. Ideally, the signal would turn green again as the next group of cars arrives. LA's traffic system overhaul went further than most others, by using cameras and sensors to measure traffic flow and make real-time adjustments to keep traffic moving.


Lomax says that poorly timed traffic signals can cause significant delays, and that cities can make big gains by updating signal timing every three years to adjust for new traffic patterns.

"If you start off with a poorly timed signal in an area that's growing and upgrade to a better-timed system that works pretty well, you can probably get rid of half the delay and in some cases maybe more," says Lomax, defining "delay" as the difference between travel times during low-traffic and peak traffic conditions.


Why Stop Lights Often Aren't Synced

But retiming traffic lights costs money and manpower. According to Texas A&M research, cities should expect to spend between $3,500 and $4,000 per intersection and devote 20 to 30 man hours for analyzing and retiming each signal. And jurisdiction issues can complicate matters when the same stretch of road passes through different municipalities. What if one city or suburb doesn't want to fork over money for the upgrade? That can hold up a retiming effort for years.

A TV news report noted that the last time traffic lights were all synced in Atlanta (another city with major traffic congestion) was in the mid-1970s when Atlanta had traffic lights at 320 intersections. In 2011, it had lights at a further 945 intersections that were not included in the coordination system.


Retiming or syncing traffic lights is only one tool in the traffic-fighting toolbox. Lomax says that another effective strategy is to clear away accidents and disabled cars more quickly. He says that commuters aren't as miffed by long drive times as they are by unpredictable drive times. What really stresses people out is when the daily 45-minute commute unexpectedly becomes an hour-and-a-half commute. And the most common causes of unexpected delays are accidents and stalled cars.

And here is one more reason why synchronized traffic lights may not help with traffic flow as much as we'd like: Once people realize that congestion has improved on a street, it encourages them to get in their cars and drive on it, thereby increasing the number of cars on the road — which means more traffic.