Cool Gus’s Resident Scientist Checks In With His First Post!
How Do Activated Traffic Lights Work?
Let’s welcome my son Craig Cavanaugh to the blog and his unique view on the world around us.
By Craig Cavanaugh:
I’ve been fortunate… and have been able to spend my life learning and dabbling. I’ve spent a fair amount of time in school and research labs but also teaching and working. I have a BS in Engineering Physics and a PhD in Materials Science and took a winding journey to this point. I interned at a biotech company attempting to adapt their fast, chip-based flu assay to detect genetically modified soybeans. I have played with high-power, ultrafast pulsed lasers and still know a bit about aligning and modelocking a Ti:Sapphire capable of generating femtosecond-long pulses. I worked as a field engineer in OK, KS, and CO. I’ve taught Chemistry and Physics and know the effort it takes to prep labs for 1000+ students every week. I can synthesize graphene in solution and coat it in platinum. And most recently, I learned how to build ion trap mass spectrometers and fabricate semiconductor-based field effect devices in a cleanroom. All of these experiences have one thing in common… somebody took a chance on me. I took every one of them seriously and learned as much as possible as quickly as possible. While some were more successful and fulfilling than others, overall I have fond memories of every single project, teacher, and coworker. Now… I’m searching for somebody to take a chance on me here in sunny San Diego. In the meantime… I’m looking into teaching and tutoring opportunities, spent some time at CodeAcademy, and acquired certificates for a microMBA and a Biotech Lab Skills, all in an effort to keep my mind fresh and ready for the next challenge.
Today… I’d like to try and blog a bit. My parents and family tend to point out that I have an interesting point of view regarding how things work. It mostly comes from my desire to know how as well as why things work and by applying my knowledge of physics, chemistry, math, engineering, economics, and reality. So… that got me thinking, the longer we live, the more random knowledge we accumulate. Everyone has a different point of view and a different set of priorities. Thus, while two people may know the exact same thing, they most likely have two completely different mental models for how or why that thing is the way it is. I’d like to write down some of the random thoughts that I have or answers to questions my friends and family have asked me. My hope is that it gives the reader the chance to see an example of critical thinking at work and maybe give them the desire to start thinking about the world around them in a more critical manner in order to feel more confident in this crazy world.
Up first… stop lights. I know starting with the exciting stuff, right? My dad has a Jeep (JK) with big tires — it’s completely kitted out and ready for the apocalypse aside from possibly changing out the gear ratios in the axles. It’s ironic, because as a teenager my inner-Tennessean was strong, and I talked nonstop about getting an XJ Cherokee and putting big tires on it. I had lists of the necessary parts (lift kit, wheels, tires, lockers, bumpers, spare tire racks, etc) and labor estimates that would successfully put me behind the wheel of a Cherokee with big tires. I spent hours poring through inventories and local sales looking for the perfect XJ (must have a manual transmission). My parents nodded and pretended to listen, and in the end (sagely) ignored this phase of my adolescence. I never did anything too crazy in the car because I always follow the rules… but I can think of a few ‘oh shit’ moments driving in the snow or rain where the raised center of gravity of a Cherokee with big tires would have resulted in a much different outcome than (get this…) an Explorer with normal sized tires. Remember the trouble that Ford got into for those old Firestone tires?
Anyways… back to stoplights. On a recent visit to Knoxville, I was driving my dad’s Jeep and we pulled up to the stoplight to get out of the neighborhood. Like most stoplights… it’s sensor actuated and only stops cross-traffic on the larger road when it detects a car waiting on the smaller road. My dad immediately expressed frustration with the light…saying that it rarely changes for his Jeep, especially on left turns. He ends up sitting there waiting until somebody pulls up behind him or hanging a right followed by a u-turn at the next intersection. Not the end of the world… but a pain in the butt.
Given that I’m a problem solver and troubleshooter, I immediately started thinking. First question… you’re sure this light has a sensor? Yes, because it changes nearly instantly when driving the other car. So, the issue seems specific to the Jeep. I wonder what kind of detector it uses? A common misconception is that stop lights register the weight of a vehicle — that there is a basically a scale buried underneath the road surface which detects the weight of the vehicle. This is a convenient explanation for why cyclists (both bi- and motor-) have similar troubles activating stop lights as the Jeep does in this case. However, it doesn’t quite add up. The biggest issue is reliability and cost. A scale would require a moving part of some sort… a spring or a liquid-filled tube. Not that it wouldn’t work, but it would be prone to failure and when you’re burying something under the road it costs a lot both in terms of money and downtime. Ideally, you want your sensor to outlast the pavement, so they can both be replaced at the same time. Let’s throw out some hypothetical numbers. What if the pavement lasts 15 years and the sensor for 5? Then, that chunk of road would (on average) need to be dug up and fixed twice before the crew comes back through and repaves the whole thing. In addition to the repair costs, the seams leftover would leave the lane susceptible to freeze-thaw damage, increasing wear, and resulting in potholes that will require frequent repairs. Not as big of a deal as replacing the sensor… but not what you want to spend money on if it can be avoided. Of course, there is a break over point, where if the sensor you are burying costs too much upfront… a city planner would rather incur the hassle of replacing a cheaper sensor and fixing potholes. But, I digress…
Fortunately, there exists a sensor that is both cheap and long lasting… a buried loop of wire. On the way back home, I noticed the grooved pavement indicating the location of the wire. This system is basically a metal detector and based upon the concepts of resonance and inductance. Have you ever tied a jump rope to a fence and shaken it? When you hit just the right frequency (speed at which you are shaking your hand up and down) it will resonate. This is observed by the formation of a wave pattern in the rope. If you increase the frequency of the shaking, you can access new wave patterns… but in between each wave the rope will have a difficult time oscillating — it will require more force for you to drive the rope until it ‘drops’ into a new resonance (wave pattern). The appearance of the patterns is dependent upon the length of the rope and can be calculated with a bit of math. A similar thing happens when you ‘shake’ the electrons in a looped wire. Put a wave generator on loop and there will be certain frequencies where it requires less effort for the generator to oscillate the electrons.
That’s great… but how do you detect a car with a loop of oscillating electrons? In the case of a wire, the resonance creates an electromagnetic field that extends beyond the boundary of the wire’s surface. This field easily passes through insulators (such as insulation and pavement). This is because it only interacts with ‘free electrons.’ Insulators keep all of their electrons bound close to individual atoms, while conductors have electrons that are free to bounce from atom to atom. Thus, when a conductor passes near a wire that is generating an electromagnetic field, its electrons begin to oscillate and absorb some of the energy from the field (a process known as inductance). In the case of the stoplight sensor… the wire buried underneath the road is happily humming along requiring say 5.0 volts to resonate. When a car drives over it and comes to a stop, the electrons in the iron atoms in the steel frame absorb some of the field and the wire now requires say 6.0 V to maintain resonance. This voltage difference is easily measured and passed onto a logic circuit which determines that it’s above a certain threshold. This threshold is necessary to ensure the light does not register false positives (i.e. the change in a pedestrian’s pocket or a large truck passing in the other direction, etc). Once the threshold is surpassed, this information is sent to another logic circuit which will change the stoplight depending upon a few variables such as the last time it switched and the time of day, etc.
So… what happens when a Jeep with big tires pulls up to the light? Well, the frame is sitting farther away from the pavement thanks to the added ground clearance afforded by the tires and lift. Granted, the axles have only been raised by the difference in tire size, but there is much less metal in the axles vs. the frame/transmission/etc. Remember that electromagnetic field being generated by the looped wire? Well, another detail is that the strength drops off with distance (actually the square of distance). So changing the distance between the Jeep and road surface reduces the interaction between the sensor’s electromagnetic field and the Jeep. The electrons in the iron atoms in the Jeep will oscillate, but with less gusto because they are farther away. This means less energy is absorbed and the wire may only require 5.1 volts to maintain resonance when the Jeep pulls up. This is likely below the ‘threshold’ setting of the light… and thus the logic circuit ignores the presence of the Jeep. Bummer.
So… now that we know why there is a problem, how do we fix it? Well, you could always hang a chunk of metal off the Jeep… but this is inelegant and likely to cause a few headaches. I’d start with a quick experiment. Approach the light in slightly different lane positions — see if there is a portion of the lane where there is enough interaction with the electrons of the Jeep to surpass the threshold of the sensor. My guess is that there is a sweet spot, and it’s probably a similar lane position for most other lights, so it would be a good habit to get into. Another solution that I’ve heard used by motorcyclists (and avid road bikers) is to place a magnet near the bottom of your frame. The magnetic fields will also interfere with the resonance of the buried wire and trip the sensor threshold despite there being far fewer electrons to oscillate in a motorcycle frame. Another (possible) suggestion is particular to lifted trucks with solid axles. I’ve seen magnetized differential covers… the idea is that having large tires is hard on the moving parts inside the differential and any metal shavings will get stuck to the magnetic cover rather than being left to circulate in the differential oil and cause additional wear. It’s possible the field strength from these covers could also be enough to trip stop light sensors… but that would probably take some trial and error (expensive for a trial) or forum searches (anecdotal information must always be taken with a grain of salt!). Alternatively, you could get the same effect by ordering a strong magnet and placing it on the existing differential cover. Just make sure to secure it so it doesn’t jostle loose on the freeway and take out somebody’s windshield.
Anyways… those are my random thoughts for today. Thanks for sticking it out this long. I hope you enjoyed.
Originally published at writeitforward.com on October 30, 2016.