Stoplight timing

October 15, 2005 9:53 PM

How does a stoplight know what time it is?

Around my house, after midnight, a bunch of stoplights will start to blink yellow, instead of operating normally. Another example are lights at certain intersections that have different timings at different times during the day. Obviously, these stoplights have a clock of some sort, but how is it set? Do they receive a signal of some type, maybe from the power lines? Or are they set once and then forgotten? If so, how do stoplights deal with daylight savings?

posted by hobbes103 to technology (14 comments total)

Stop lights are do not have internal logic controls...all the switch timings and other "logical" functions of the lights are controlled from a box mounted nearby. This eliminates the problem of costly repairs to the light pole when it's hit. Next time you're at an intersection, look for refrigarator sized box nearby. That has the controls in it. I assume the lights have an internal clock that gets it's time data from the phone company. (otherwise, they'd have to reset the clocks in each light every time the power went out.)
posted by cosmicbandito at 9:59 PM on October 15, 2005

Where I live, they're not automated. The police switch them over using the control boxes at nine or ten o'clock.
posted by cribcage at 10:00 PM on October 15, 2005

In modern systems, they're computer controlled, and a terminal in the municipal building can change any light's behaviour at any time.
posted by I Love Tacos at 10:05 PM on October 15, 2005

cosmicbandito pretty much has it. Most modern traffic signals are controlled by a whole heap of electronic equipment that is housed together in a cabinet at each intersection. The main thing that controls the timing of the signals is called the traffic signal controller (amazingly enough). It's about the size of a toaster oven. It can be programmed by hand or it can be sent timings and other data over a local network if it is part of a signal system. If it is a standalone intersection (not connected to any others), the clock in the controller handles any time-based control and is usually kept "on time" by the power cycles feeding the cabinet (60 cycles/sec keeps the clock more or less on time). If it is networked it is probably synced daily to a central clock.

As for daylight savings, the software in the controller usually can deal with that by being programmed with the daylight savings start and end dates and it can adjust its clock accordingly.
posted by contessa at 10:36 PM on October 15, 2005

[Followup Question: Can one hack the data signal, custom-programming the light patterns? I'm thinking that with GPS tracking, Google maps, and some custom glue, I could always have green lights!]
posted by five fresh fish at 12:01 AM on October 16, 2005

All green lights? no problem!
posted by I Love Tacos at 12:13 AM on October 16, 2005

When I was about 4 years old, I thought that traffic lights had -- believe it or not -- babies in them...and the babies would push colored buttons corresponding to the red/yellow/green that drivers saw. When someone got stuck at a red light, it meant that the baby had fallen asleep, and honking a horn would startle the baby awake and result in another button being pushed.

Weird stuff that kids imagine, eh?
posted by davidmsc at 12:22 AM on October 16, 2005

Eagle TCS are probably the most commonly found devices in the control boxes. They can be interfaced on site with laptops running their special software to modify the timing or many times are networked together with cable running to the city hall. It's doubtful you could hack them without being onsite and breaking into the control box.
posted by JJ86 at 12:49 AM on October 16, 2005

Why couldn't they just get the time over the radio? I don't know about the US, but in Europe you can buy clocks which are always in perfect time, as they receive radio signals telling them what time it is.
posted by wackybrit at 6:01 AM on October 16, 2005

Wackybrit, we have those here, too. Your question has me wondering, though, about the technical feasibility: are there places in the US that are so building-dense that radio waves would not be reliable, or alternately, are there places so remote in the US that radio waves wouldn't make it there (but that are still populated enough to justify traffic lights)?
posted by kimota at 8:07 AM on October 16, 2005

davidmsc: We do think funny things - when I was little, I thought birds were in the control box changing the lights. My friend and I were riding our bikes around the block and saw a hurt bird next to the pole the box was on. The next time we came around, the bird was gone, and we heard a knocking sound come from the box. And somehow we managed to connect the knocking to the changing of the traffic light.... and the bird. I still think that every time I hear the knock-box.
posted by dpx.mfx at 8:12 AM on October 16, 2005

The next time we came around, the bird was gone, and we heard a knocking sound come from the box

Still there, probably. The knock is the sound of a relay being triggered. Relays are electrically controlled switches.

Say you want to drive a bank of lights from a microprocessor. Problem: Bank of lights needs lots of power, microprocessor can only output a bit. Answer (one, at least) -- a relay. Microprocessor powers an output line, this engages a relay, which switches a more powerful line that turns the lights on.

Many relays are magnetic reed based. The switching power energized a coil, which pulls the reed closed. Small relays go "click" when this happens, large ones go "thunk." Part of the noise is the arc when the switch arm gets close enough that the airgap can't isulate, this is more critical as the voltage increases, and very high voltages wear out relays in short order.

There are modern solid state version that don't have the mechanical issues with traditional relays, but when your moving a bunch of power, the mechanical ones tend to be the only answer. Traffic light circuits tend to draw a fair amount of power. At minimum, you need to light 2 rather bright lamps, that take a fair amount of power.

There's also the clever interlock system that, if both sets of greens try to light, either kill the entire tree or fault to blinking red in all directions. 4 way solid red isn't good (it closes the intersection) but 4 way green is deadly, and a fair amount of clever goes into making sure that can't happen, and if it does happen, the box either faults or shuts down. This is why the rest of the lights are blinking if a traffic light in a set is knocked over. Once service personell are on the scene, then can put a dummy load in for the downed pole, and if there's still enough visible signals, reset the controller.

In most states, you have to treat a stoplight that is showing no signal as a stop sign. Exception is a "bagged" signal, that's treated as a no-signal present. Usually, though, there's something else there, like a stop sign.

Radio clocks: Yeah, the US has one as well, WWVB out of Fort Collins, CO. Until recently, though, decoding clocks weren't cheap, and syncing to the 60Hz phase of the power lines was accurate enough and trivial to do. Nowadays, more and more remote controllers sync via WWVB, or if networked, they use the NTP protocol. In very recent years, GPS chips are cheap enough that they can provide a cheap and very high quality time signal, if you need such.

Finally, in many cases, the controls are in an underground vault, rather than the big box on a pole -- often in intersections where the big box would be vulnerable to being hit, or on simple intersections not requiring full sized controls.
posted by eriko at 9:26 AM on October 16, 2005

Lots of places used a QNX setup to control their traffic systems
posted by srboisvert at 11:42 AM on October 16, 2005

A somewhat related question and thread here
posted by bunglin jones at 8:20 PM on October 16, 2005