It’s About Time: How traffic signals keep the Rocket City moving

single-meta-cal August 6, 2021

Traffic signals tell drivers when to stop or go, but few people give them much thought beyond that. It’s only when they’re either flashing or stuck on red that drivers tend to notice them at all.

Huntsville’s signals might be more concerning if they weren’t part of an intricate system designed to ensure optimal safety and mobility for drivers and pedestrians. For a better understanding, Nicholas Nene, P.E., deputy director of Traffic Engineering, pulled back the cover ­– er, pavement – on the stop-and-go world of traffic signals.

Picking up the signal

It’s hard to know how signals function without first understanding how they work. Traffic signals might look the same, but they all rely on a mix of technologies for vehicle detection.

Traffic moves along Madison Street at the intersection of Gates Avenue in downtown Huntsville

Traffic moves along Madison Street at the intersection of Gates Avenue in downtown Huntsville.

Huntsville’s signals use a combination of radar, loops and cameras. Nene offered a brief synopsis of each:

  • Radar: The most reliable (albeit most expensive) type of system is radar, which continuously tracks a vehicle’s speed, range and estimated time of arrival at the signal. One above-ground unit is typically installed at a large intersection because it can detect multiple lanes.
  • Loops: This form of detection operates below the road and works like a metal detector. When a vehicle passes over a loop sensor, the loop field changes and senses the presence of an object. Inductive loops and other detection devices like axle sensors are often used to determine traffic counts.
  • Cameras: Overhead cameras on top of signals are focused on vehicles and not drivers. As vehicles enter the camera’s field of view, it detects a change in zone and sends a signal to the signal’s controller.

A matter of time

Traffic signals play a substantial role in moving people around the City. But, like most things in life, timing is everything.

Nene put it simply: “Signal timing is a process to ‘optimize’ the operation of signalized intersections.”

But why’s it so important? Signal timing reduces stops and prevents snarled traffic at intersections in all parts of the City, reducing commute times. Mayor Tommy Battle has long strived to ensure 15-minute commutes for citizens, no matter where in the City they reside.

Signal timing also steadies traffic patterns due to sudden changes from growth or new development. For example, when Cecil Ashburn Drive was closed for improvement, Governors Drive absorbed the traffic. Signal timing was critical to ensuring a smooth flow of traffic for those extra vehicles.

From a cost-benefit analysis, signal timing is one of the most cost-effective ways to improve traffic flow and safety.

Research and science

The City’s signals are managed through the Centracs Advanced Transportation Management System, which allows Traffic Engineering to easily make real-time cycle length changes through an intuitive software system. A cycle length is the total time to complete one sequence of movements around an intersection and can vary from 60 to 180 seconds, depending on capacity or if it’s a major roadway.

The adjustment process might seem simple enough but there’s a lot of thought and planning that goes into making those changes. For example, have you ever found yourself among other vehicles able to freely pass through several consecutive signals without stopping? That’s not just luck; it’s by design.

Have you ever noticed some signals change faster or slower throughout the day? That’s also by design. Nene explained cycle length times increase or decrease depending on the time of day.

One of those making timing decisions is a dedicated signal timing specialist, who evaluates and adjusts signal timings every 12-18 months or as traffic demand changes.

Improving flow and safety

“This position has an in-depth knowledge of demands for the entire City of Huntsville’s roadway network,” Nene said. “In field observations, acquiring updated traffic counts and a greater understanding of the area that each signal or signalized system is surrounded by are part of the information that is required.”

Nene said evaluations are needed when new signalized intersections are added near existing signalized intersections. Traffic Engineering regularly fine-tunes corridors to ensure ideal timing and efficiency for all residents, no matter how they get around.

“The objective of Traffic Engineering is to respond to the demands of all types of motor vehicles, bicycles and pedestrians in an optimum or balanced manner,” Nene said. “From a cost-benefit analysis, signal timing is one of the most cost-effective ways to improve traffic flow and safety.”