Editor's note: See the author's other related article: Traffic lights, the hated holiday lights
If there is one thing in human society that appears to lack intelligence, it’s a traffic light. As I researched this subject, I came across many references regarding how traffic lights are controlled. In addition, there were references to the logic associated with traffic lights. As you may have already guessed, traffic lights have sparse if any intelligence and certainly don’t maintain pace with advances in technology….at least from a control standpoint. The lights themselves have made advances as LED technology moves forward. Lights will be addressed in a separate blog. Computerization has benefitted traffic in general, however there is sparse reference as to how. As with many technologies, the US seems to be lagging when it comes to sophisticated traffic control. Therefore this blog, like traffic control itself, is more about the lack of technology.
Traffic lights are mostly controlled in two basic ways, by sensor detection or timers. Within these two control methods there is very sporadic computer control. For the most part, traffic lights are on their own, especially in the US, the world’s most advanced country….or at least we were until we gave away technology and education to the world.
As Wikipedia puts it, “A variety of different control systems are used to accomplish this, ranging from simple clockwork mechanisms to sophisticated computerized control and coordination systems that self-adjust to minimize delay to people using the road2 .”
The earliest form of traffic control was in the lights themselves. Lights had to have failsafe mechanisms for making sure they control the intersection properly. According to Reference 1, “the system was configured so that conflicting signals were impossible”. In other words, two green lights at the same time work for oncoming traffic on the same road, however not so much for traffic approaching from the other street in the intersection. Even then, coordination between arrows and advanced green lights is resident in the traffic light itself. As lights got more sophisticated, so did the resident intelligence.
Timer systems are just that, they permit a certain light to be on for a certain amount of time. Moving from the resident control within the individual traffic light to the control of many lights; the effort develops into a system at the geographic level. For example, in many cities these lights actually are coordinated to deter traffic, “Congestion can often throw off any coordination, however. On the other hand, some traffic signals are coordinated to prevent drivers from encountering a long string of green lights. This practice discourages high volumes of traffic by inducing delay yet preventing congestion2 ”. There you have it. Progress is hindered on purpose. If only progress were enhanced as much as hindered.
Wikipedia breaks ‘timed system level control’ down into two categories, the older synchronized systems and the more modern coordinated systems. Without going into specifics, lights either cascade in a coordinated event as you travel or all turn the same color at the same time in synchronization. The desire is to hit the so called “green wave” whereas every light is green as you encounter it. Timer systems are common in high traffic areas such as cities.
Detection systems operate where light side traffic requires only the occasional triggering. However, there are detections in many cities as well. This is how green arrows and advanced greens know when to activate.
Timing systems are slowly becoming solid state. Early versions were electro-mechanical employing dials, shafts, and gears2 . These electromechanical arrangements cover durations from thirty five to one hundred and twenty seconds or, if you will, two minutes. Understandably, solid state is programmable much more than are gear arrangements, unless of course a transmission of sorts is used which adds complication and cost. Moving parts also have more of a potential to wear out. Also, in the electro-mechanical mix are relay-based arrangements. The lower power, programmability, and higher reliability of solid state is beneficial to traffic control and thus is slowly being implemented.
Detection systems trigger a light based on the presence of a vehicle. Detection uses several methods including air filled hoses3 , treadle switches2 , lasers3 , and the most popular, inductive coupling2 .
Treadle Switch [image courtesy of Wikipedia2 ]
Although it would appear that the weight of a vehicle might be a method of sensing presence, the closest detection arrangements are air filled hoses and treadle switches. Perhaps this is due to having to detect everything from a bicycle to a loaded semi-truck. The large range of weight variation, along with the method to implement and connect, makes weight detection difficult. Piezo-electrics are a common method for converting pressure or weight changes into electrical impulses. In the sixteen popular search responses, no reference to piezo electric detection emerged. This common electo-mechanical interface seems to be absent in traffic control.
Inductive coupling acts like a big transformer. A loop or even a sawtooth wire arrangement is laid into the road surface. When a vehicle is over the inductor, it changes the flux, which in turn changes the inductance value. Detection circuitry detects the change in inductance then signals a light change.
Sawtooth Inductor Wire [image courtesy of Wikipedia2 ]
Inductive detection doesn’t always work especially for bicycles. Wikipedia2 offered this insight on bicycles and inductive detection, “A small bicycle symbol is often marked on the pavement to inform the cyclist where to stop in order to actuate the signal. Other places simply place an additional pedestrian button near the curb where a cyclist can reach it”.
I remember one time where I was in a truck that had a suspension lift to a point that the body was outside the effective flux range and too high to trigger the light. We sat there forever waiting for the light to trigger. We were in the left lane and couldn’t turn right or run the light due to officer unfriendly sitting on the opposite corner. As I recall, we ended up backing out of the intersection and then approaching in the right lane where we could make a right on red.
Laser technology allows for the most sophisticated form of detection. Lasertech4 is a company that offers a “rapid and highly accurate laser to collect every vehicle’s speed, height, length, and traveling distance between other vehicles” as well as lasers for triggering. Triggering is similar to the trigger on your automated garage door where detection is based on breaking a beam. Many a red light camera has registered a violation when a vehicle broke the laser beam by nosing forward too far at an intersection. Even though the vehicle is shown as stopped in the photograph, a violation occurs when the vehicle is over the line. Judges rarely accept the argument of fully stopped versus vehicle position. Part of this is most likely due to revenue streams and part could be due to safety for infringing too close to the intersection. The solid white line is there for a reason. It’s not like a Boston rental car bus driver put it when referring to a stop sign as “merely a suggestion”.
Other forms of control include sound-based systems and manual control. Manual control enables an officer to take command of a traffic light. When we towed floats to the start of the Fiesta bowl parade5 , they simply used a roving motorcade of motorcycle policemen much the way they do for dignitaries and funerals. We went through the lights regardless of the color as the goal was to keep us moving. Sound based systems include triggering lights based on an approaching siren. One has to wonder if the detection has kept abreast of the many forms of sirens that are presently used. If so, detection filters would have to cover a number of frequencies in the sound spectrum range.
As you may have guessed, vehicle activated detection plays a role in sound based traffic signal control. This next excerpt tells you a funny little story about sound detection7, “An inventor named Charles Adler Jr. had the idea of inventing a signal that could detect vehicles’ honking and change signals accordingly. A microphone was mounted on the pole of the signal, and once the vehicle reached the signal, all it needed to do was honk and the light would change. However, this obviously led to the problem of unnecessary honking of vehicles, which created a harrowing experience to those walking by and those living near the signal. Thus, the honk-sensitive lights had to be abolished.”
Of course there were references to interaction between vehicles and the traffic system. With the amount of onboard communication links such as WiFi, Bluetooth, and Onstar, one would think that systems would be in place for signal to vehicle interaction. However, it seems as though these niceties are more involved in entertainment and personal communication than adding intelligence to the mix. Furthermore, the crowded frequency spectrum and lack of ability to lock on to a signal in less than milliseconds6 means that these added loads really have no place to go in terms of transmit/receive bandwidth. The frequency spectrum is overcrowded enough while more keeps getting added to it all of the time.
As with any infrastructure that is government controlled, traffic is one of the last areas to receive upgrades even though it’s one of the first and worst things to greet you on a daily basis. I welcome reader feedback as to how intelligent control is working because to me, it’s not obvious. I seem to be waiting at a lot of traffic lights. It doesn’t seem like two minutes, it seems more like two minutes underwater. And, of course, I cherish the moment when I have to remove a five hundred pound combination of a transmission and transfer case to replace a three hundred dollar clutch assembly.
- First electric traffic signal installed, history.com website, no other information provided.
- Traffic light control and coordination – Wikipedia
- How does a traffic light detect that a car has pulled up and is waiting for the light to change?
- Traffic Management, lasertech product website
- Beneath the Beauty; What’s it Really Like Inside a Parade Float #parade #float #monstertruck
- The Drone Dilemma: Lack of Bandwidth & Signal Strength, Scott Deuty, 12/10/2015 08:30 AM EST, EETimes website
- The Evolution of Traffic Lights, deegee beaconlamps.com