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
It’s a combination of sensors, timers and direct control.
Major intersections generally have sensors at the first and sometimes second queue position to detect if cars are there and need to change the lights.
In some peak tines, the intersection is generally set to specific timing, or can be controlled by the city wide traffic control inordrr to give more time to certain directions.
In off peak times (generally late at night) the intersection lights stay in the direction of major travel, and only change if there is a vehicle detected, or the pedestrian crossing button is pressed.
In emergency situations, traffic control for whole regions can be manually overridden.
It’s a combination of sensors, timers and direct control.
Major intersections generally have sensors at the first and sometimes second queue position to detect if cars are there and need to change the lights.
In some peak tines, the intersection is generally set to specific timing, or can be controlled by the city wide traffic control inordrr to give more time to certain directions.
In off peak times (generally late at night) the intersection lights stay in the direction of major travel, and only change if there is a vehicle detected, or the pedestrian crossing button is pressed.
In emergency situations, traffic control for whole regions can be manually overridden.
At most lights, there will be a seemingly out of place metal box on/near one of the four corners. This is the control box. Once either the timer expires, or the magnetic sensors (thought this was pressure for forever lol) that others have described say it is time to change, the light will change from green to yellow, hold for 3-6 seconds, then go to red. After all lights sit red for a few seconds, the other direction turns green. The sensors/timer for the other direction will let the controller know exactly which lights need to turn green. Sometimes opposite direction left turns only go green, other times all lights in one direction change, sometimes there is no dedicated left turn signal.
Having the control box right at the light allows authorized personnel to override the base programming depending on conditions. For example, the streets/exits leading away from a concert venue normally wouldn’t stay green for very long since traffic is normally pretty light. But whenever there’s an event, the basic programming can be overriden to allow a much longer green to keep traffic moving away from the venue.
At most lights, there will be a seemingly out of place metal box on/near one of the four corners. This is the control box. Once either the timer expires, or the magnetic sensors (thought this was pressure for forever lol) that others have described say it is time to change, the light will change from green to yellow, hold for 3-6 seconds, then go to red. After all lights sit red for a few seconds, the other direction turns green. The sensors/timer for the other direction will let the controller know exactly which lights need to turn green. Sometimes opposite direction left turns only go green, other times all lights in one direction change, sometimes there is no dedicated left turn signal.
Having the control box right at the light allows authorized personnel to override the base programming depending on conditions. For example, the streets/exits leading away from a concert venue normally wouldn’t stay green for very long since traffic is normally pretty light. But whenever there’s an event, the basic programming can be overriden to allow a much longer green to keep traffic moving away from the venue.
At most lights, there will be a seemingly out of place metal box on/near one of the four corners. This is the control box. Once either the timer expires, or the magnetic sensors (thought this was pressure for forever lol) that others have described say it is time to change, the light will change from green to yellow, hold for 3-6 seconds, then go to red. After all lights sit red for a few seconds, the other direction turns green. The sensors/timer for the other direction will let the controller know exactly which lights need to turn green. Sometimes opposite direction left turns only go green, other times all lights in one direction change, sometimes there is no dedicated left turn signal.
Having the control box right at the light allows authorized personnel to override the base programming depending on conditions. For example, the streets/exits leading away from a concert venue normally wouldn’t stay green for very long since traffic is normally pretty light. But whenever there’s an event, the basic programming can be overriden to allow a much longer green to keep traffic moving away from the venue.
Not what you asked specifically, but interesting nonetheless. A light stays yellow one second per every 10 miles per hour of the posted speed limit. So, on a 45 mph road, the light will be yellow for 4.5 seconds.
That knowledge has been very beneficial to me at times when I’m trying to decide to gun it or stop.
Not what you asked specifically, but interesting nonetheless. A light stays yellow one second per every 10 miles per hour of the posted speed limit. So, on a 45 mph road, the light will be yellow for 4.5 seconds.
That knowledge has been very beneficial to me at times when I’m trying to decide to gun it or stop.
Not what you asked specifically, but interesting nonetheless. A light stays yellow one second per every 10 miles per hour of the posted speed limit. So, on a 45 mph road, the light will be yellow for 4.5 seconds.
That knowledge has been very beneficial to me at times when I’m trying to decide to gun it or stop.
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.
There is one controller for the entire intersection. In simplest case, its just cycling through preset states on a timer.
The complexities happen when there is need to modify that pattern on the fly. Syncing with other intersections across the city, passing emergency vehicles, buttons for pedestrians, induction sensors for cars or busses, active citywide control to unravel a traffic jam, the sky is the limit how complicated you can make it.
Latest Answers