Exploring the science behind traffic lights
Jack Gillon couldn’t wait to show me his handiwork. The intersection of Granite and Hancock streets in downtown Quincy was our starting line; the Department of Public Works on Sea Street nearly a mile away was the finish.
“So, we’re going to get back to your office from here hitting all green lights?’’ I asked, dubiously.
“Yeah, pretty much,’’ Gillon said.
For more than an hour, Gillon, Quincy’s traffic engineer, had gushed about the glories of modern traffic lights. Thanks to semiactuated intersections, dilemma zones, loop-detector amplifiers and the like, traffic can be zipped through a city like never before, he said.
But to truly appreciate the science of traffic platooning — synchronizing long stretches of traffic lights to work in unison — we had to take a drive.
When our light turned go, I began to count. One green light. Two green lights. At the third intersection, we barely squeaked past the yellow. “I knew that was coming,’’ Gillon said, unfazed.
We slowed for someone in a crosswalk, which I was sure would kill our chances. Not so: four green lights. Then five!
A jaywalker on the next block did do us in, however, so the count ended there. But I still was impressed.
“So, if people get all those green lights at once, that’s by design? It’s not, ‘Oh, I guess I just lucked out,’ ’’ I asked.
Gillon chuckled. “Fun stuff, isn’t it?’’
Most modern traffic signals don’t operate blindly; they actually respond to the number of cars entering the intersection from various directions. There are several ways traffic lights can “see’’ cars: radio waves or microwaves can be bounced to detect moving objects; cameras can perceive changes; cars can pass over pressure-sensitive plates. But often, traffic lights know how many cars are in an intersection thanks to sensors placed strategically under the pavement.
It works this way: Picture the first car in line at a red light. About an inch underneath the pavement where the car’s idling, an electrically charged wire has been spread in a 6-by-6-foot loop to form a magnetic field. Whenever a car passes over the loop, it disrupts the field.
A record of every disruption is sent to a control box, a metal cabinet about 4 feet tall positioned nearby. You may have never noticed the boxes, but Gillon assured me they exist at just about every signaled intersection around here. A tiny computer inside the control box analyzes the disruptions. If they keep happening, it means that cars are still entering the intersection from that direction, so the light should stay green. If the disruptions stop, it means there are no more cars and the light should switch to red.
A traffic light can’t stay green forever, so there’s usually a maximum number of seconds allowed before the computer switches the light to red. On a main street, it might be 30 or 40 seconds, Gillon said. But with no vehicles coming, the light might switch to red in half that time.
If it’s an abnormally long disruption — lasting minutes, not seconds — then Gillon knows there’s probably been an accident or a breakdown at the location. If traffic lights are stuck on red, he knows there’s probably an equipment malfunction.
With such technology, he’s able to synchronize traffic signals at one intersection with those farther down the street, allowing a whole string of cars — a platoon — to move as a group from green light to green light.
“When I was in Brookline I had cars going in to Boston in the morning and cars coming out of Boston in the evening. The system detectors showed which platooning program I should play: an outbound or an inbound one,’’ he said. It could also change on the fly: after a Sunday afternoon
By platooning groups of cars, fewer vehicles get stopped at red lights, reducing idling emissions by as much as 20 percent, Gillon said. Extending green lights to allow 18-wheelers to pass through intersections — which Quincy does by use of surveillance cameras — likewise helps the environment.
Fewer cars stopped at intersections also can mean fewer rear-end accidents, as drivers’ attention tends to drift when sitting at red lights, Gillon said.
In the future, Gillon hopes to adjust his system to hold green lights for MBTA buses that are running behind schedule.
Best of all, with efficient traffic lights, everyone can get where they’re going faster.
“You’ve probably got 35,000 or 45,000 cars coming through here a day,’’ said Gillon, stopped at an intersection near the end of our journey. “If you’re saving even six seconds for all of them, that’s a lot, right?’’
Peter DeMarco can be reached at firstname.lastname@example.org. He also updates a Facebook page, “WhotaughtYOUtodrive?’’