How traffic signals work

October 24, 2014 | 0 Comments

There’s an intersection you’ve never really noticed because its light always seems to be green when you blow through it, and there’s another that seems to have the longest red light in the world. Both experiences reflect intentional differences in the ways traffic signals work, and while it can seem like cars trump pedestrians and cyclists in traffic-signal design, it’s safety, and not one transportation mode over another, that acts as the governing principle.

traffic signal timing

This diagram shows traffic signal timing on Oak Street in Oakland, CA. From Eric Fischer.

Road width, time of day, number and type of pedestrians—are they more likely to be senior citizens or schoolchildren, for instance—are other factors affecting how long a car or cyclist may idle or speed through an intersection, but three elements define each signal point: a controller, traffic-light heads, and detection.

A controller—the brains of a traffic signal, if you will—holds the information, such as traffic demand, that influence the sequences by which lights may run. For instance, traffic lights at some intersections may operate on prearranged, regular intervals, regardless of demand. Called fixed-time intersections, they require no detection devices because variables such as traffic build-up make no difference to the timing of the signals. Such intersections are commonly found in larger cities, where traffic is constant at all hours of the day.

Intersections that track detection are described as actuated and are more responsive. They can feature devices, such as roadway sensors called inductive loops that indicate when a vehicle is approaching. They relay information like how many cars are waiting at an intersection or when one has entered a turn lane. The controller uses this information to determine actions, such as activating the arrow light for a turn lane or setting the length of a light. If there’s traffic build-up, a controller may hold a green light for longer to ease congestion.

Actuated intersections don’t apply only to cars. Some feature crosswalks with buttons that allow pedestrians to stop the flow of vehicular traffic so that they may make it safely through the intersection. Described as a demand-dependent approach, they ensure traffic stops only when pedestrians are present, but the approach can be used for vehicles, too—on a minor road that intersects an arterial roadway, for example. When sensors on the more lightly trafficked road detect a vehicle’s approach, the controller can adjust the lights along the major road to allow the car to smoothly join the traffic flow of the arterial.

The Array of Things, an initiative spearheaded by the University of Chicago’s Urban Center for Computational Data (UCCD), may soon provide engineers with more data to enhance pedestrian traffic flow. Forty sensor nodes will be added to light poles in the Chicago area later this fall and, in addition to tracking information like temperature, carbon monoxide levels, and sound, the Bluetooth-embedded sensors will be able to count the number of pedestrians. As people stroll past, the modem would note each person’s smartphone and identify it as a pedestrian.

“You have this situation where you might have 150 people on the street corner waiting for a walk light, and there’s only one car that needs the green light,” said Charlie Catlett, director of the UCCD. The sensors would provide an argument for shifting traffic priorities to pedestrians. Currently, Chicago “is set up to bias its operations toward vehicles,” he said.

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Category: Infrastructure, municipal