Traffic lights are so ubiquitous that we hardly give them a second thought, except to curse their existence when they impede us on our daily drive. But no matter how much it seems like traffic lights have the ability to read our minds and tell when we’re running late, they’re really not much more than a set of lights and a programmable controller. Simple in practice, but as usual, the devil is in the details, and for a system that needs to work as close to 100% of the time as possible, the details are important. Let’s explore the inner workings of traffic signals.
The traffic lights and crosswalk signals at an intersection are only the public user interface, of course. The interesting stuff is going on in the control box. There’s at least one at every intersection, usually a plain metal cabinet set back from the road, sometimes camouflaged with public bills or graffiti. But inside are the guts of what makes an intersection work and keeps vehicle and foot traffic moving smoothly and safely.
Unsurprisingly, most traffic signal controls started out as purely electromechanical devices. Cabinets were chock full of synchronous motors turning timing wheels with cams to cycle the intersection’s lights through the proper sequence. One old time controller that was common up until recently was made by Econolite, and the insides are a paragon of sturdy design.
The controller shown in the video is a pretty simple one for a straightforward intersection that only needs to run a fixed program or two plus allow for a timed switch to a “flash only” mode after a certain time of night when traffic drops off. More complicated electromechanical controllers implemented crosswalk signals coordinated with the traffic signals, and provided for magnetometers or pressure switches installed in the pavement to detect the presence of vehicles and alter the timing of the signals.
A System of Interconnected Things
Electromechanical controllers served municipalities well for decades, and some intersections are likely still controlled by one of these simple but robust devices. As technology advanced and traffic engineers came up with more and more complicated intersections to support increased demand, the old electromechanical boxes were retired in favor of solid-state controls.
The first generation of solid-state controllers began hitting US streets in the mid-1970s and provided for more sophisticated control and better safety. Modern controllers have to implement a conflict monitor unit (CMU) to detect any failures of the program and resolve them safely. When all else fails, the CMU will switch the intersection to an all-flash fail safe mode so that all traffic has to stop.
Leaving a major intersection on all-flash is obviously not something municipalities want to go on for very long, so networked controllers came along that could phone home for help. Networked controllers also offer the potential for coordinated control, where a central control system capable of monitoring sensors from multiple intersections can override the program at any intersection. Coordinated control has the potential for greatly increasing the carrying capacity of roads, but it’s not without its costs; like any other networked system, coordinated control systems are targets for attack.
There’s much, much more to the story of how traffic systems are automated, and our own [Bob Baddeley] will be taking a deeper dive in his article later this week. But before this introduction closes, let’s talk about preemption.
One interesting feature seen in some modern intersections is emergency vehicle preemption. These systems use a sensor to detect an approaching emergency vehicle and switch the intersection to all-red except for the direction the vehicle is approaching from. Most preemption systems in the US seem to use optical triggers; the emergency vehicle will have a front-mounted IR transmitter, while the intersection will have a receiver mounted near the traffic lights. The vehicle transmits a modulated signal to the receiver, which signals the controller to preempt the normal intersection program. Newer preemption controls use GPS to locate a vehicle relative to an intersection to determine if preemption is required.
Can I Do That?
No matter what color hacker hat you wear, thoughts can’t help but turn dark with the possibilities presented by preemption IR receivers dangling temptingly over busy intersections. Is it possible to zap an intersection into preemption with something as simple as a programmable IR remote? There are a ton of videos out there where teenage doofuses purport to do just that, and while we’re not saying it’s impossible, color us skeptical. Most videos show a programmable remote or smartphone with a Morse code flashlight app being used at intersections that clearly aren’t preemption-equipped. Dubious at best.
IR preemption signals from an approaching emergency vehicle could plausibly be captured by a digital camera and potentially be decoded for later playback. But there are a few reasons this is unlikely and not widespread. Preemptions are generally reported to a central station and would no doubt raise suspicions if one occurred where no emergency vehicle is expected. Anyone misusing the system would face federal charges of tampering with a traffic signal. And these systems do fail-safe, stopping all but one direction of traffic rather than actually allowing an attacker more control.
So next time you’re fuming as you’re stuck at an eternally red light, just relax and think about the how it’s all controlled — and pray for a fire truck to come up behind you and change the light legally.