Did You Know – Traffic Signals, Part B

Posted by on Mar 24, 2015
Emergency vehicle preemption device that receives the optical signal from the emergency vehicle.

Emergency vehicle preemption device that receives the optical signal from the emergency vehicle.

As civil engineers and surveyors, there are many aspects of our daily jobs that are second nature to us. But we hear common questions about our field often from the public. In this “Did You Know” blog series, we’ll answer some of these questions.

Last week, we answered 5 common questions we hear regarding the basics of traffic signals, like how they detect vehicles, how they are programmed and why they are programmed in certain ways. In the second part of this “Did You Know” series post, we tackle a few more specific questions about traffic signals.

Which routes in the Birmingham area have coordinated lights? Why not more of them?

There are too many routes in the Birmingham area with coordinated traffic signal systems to name them all. A few examples are US-11, US-31 and US-280. Coordination of signals is often recommended when the distance between signalized intersections is one-quarter of a mile or less. Signal timings need to be updated regularly as traffic volumes increase or decrease over time. Every two to five years is a recommended practice. A very common reason that there are not more coordinated signal systems, or updates to existing coordinated systems, is lack of experienced manpower to keep up with ongoing maintenance.

What are the flashing yellow arrows that are starting to show up around the state?

The flashing yellow arrow (FYA) is a type of signal display for left turns which is intended to replace the five-section signal display. FYA displays are new to Alabama. Studies have shown FYAs to be more effective at conveying when left turn movements are protected or permitted. FYAs also eliminate the “yellow trap” issue, a condition when a driver attempts to complete a left turn under a circular yellow when it is potentially unsafe to do so, not realizing that opposing traffic still has a green light.

What is the device that fire trucks use to change a traffic light to green?

Emergency vehicles can be equipped with devices which interrupt, or “pre-empt” the normal operation of a traffic signal when activated and change the signal to green for the emergency vehicle. This is known as emergency vehicle preemption (EVP). In our area, EVP technology is typically based on optics (light), acoustics (sound) or GPS. EVP devices alone do not change the traffic signal. An EVP receiver device mounted somewhere on the signal wire or arm will receive a signal from the emergency vehicle and send it to the controller, which has been programmed to activate the green display for the emergency vehicle upon receiving this signal.

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