I don’t understand how they work, what they’re hooked up to and how they know when to turn from green, to yellow, to red. Are they timed? Is there a sensor, perhaps a small man hiding in the pole controlling it?
In: 7
In the lanes in front of a light, you will often see a wide, thin circular groove carved into the pavement. There will often be more than one, placed roughly where each car in line would stop if they were waiting for the light.
These grooves hold a large hoop of wire with a small bit of electric current in it. (Sometimes it’s hidden under the pavement better and you can’t actually see it.) The current in the wire bounces back and forth in a set time. However due to the laws of physics, if a large bit of metal is sitting over the wires (like a car), the current will bounce back and forth with slightly different timing. This is how the light knows a car is stopped there, which lane it’s stopped in, and how many cars are waiting.
Many people think these are “pressure“ sensors, but that would require the sensor and the pavement itself to actually have moving parts, which would be too fragile and expensive to maintain over time, and most cars’ wheels would “miss” them anyway.
From this information, the light is programmed to give preference to the cars waiting in one direction versus others, how long it should hold that light green for, and everything else. It might even vary by time of day, depending on which direction of traffic has the most cars at rush-hour or similar.
Or sometimes, especially in a rural area where the traffic does not need to be controlled so tightly to avoid backups, there are no sensors at all, and the light just runs on a simple timer to save money.
Latest Answers