Why do traffic lights have a blink cycle?

June 23, 2008 11:49 AM

Why do traffic lights switch to flashing red after a power outage?

I don't get it. We've had lights go completely dead in town the last couple weeks during storms - no blinking. We've also had plenty of flashing reds, which I assume occur after the power has been cut, and then restored.

Why do the lights not simply go red in all 4 directions for 30 seconds and then begin a normal light cycle again?

I googled around, but didn't find much by way of a helpful answer.

posted by ChrisManley to travel & transportation (15 comments total)

I was under the impression that 4-way blinking red lights was the failsafe mode for intersections that have some sort of backup power. taking the sensors (if they exist) out of the loop uses less power.
posted by pmbuko at 11:55 AM on June 23

Perhaps as a way to signal to someone that the light has previously had a power interruption and may need service.

It seems to me that flashing red is the safest default, as it causes everyone to stop. While you may be able to easily tell that normal operation would be warranted, there's no reason why that would always be the case. (One street could be blocked by a giant tree or something that someone focussed on the green light might not see until too late.)
posted by OmieWise at 11:57 AM on June 23

You're probably assuming a level of sophistication in the traffic lights that simply isn't there.
It's possibly that the traffic light system isn't connected to the other traffic lights and that the system doesn't have any flash memory that can exist past a power outage.

Or think of it as a cover your ass protocol. If we lose power, lets assume that a few traffic lights are going to die or go on a rampage, so we'll set a simple safety function that puts them in warning mode until we get a physical person there to check it and reprogram the timing sequences.
posted by Static Vagabond at 12:00 PM on June 23

a lot of these things are controlled by switches or computers - the controllers may not come back up immediately after a power outage, so they fail into a four-way stop. (see Wikipedia.)
posted by mrg at 12:00 PM on June 23

They don't all do it. We have some around my area that blink red and yellow, depending which way come at them. Heavier traveled road gets the yellow and lighter traveled road gets the red.
Why they don't resume their normal program after restoration of power is a mystery. You would think that once power is restored, you would want to get things normalized asap. But, maybe it requires a DPW guy to come buy and press the magic button or something.
Just another one of life's mysteries.
posted by a3matrix at 12:01 PM on June 23

In most communities, the traffic light timings are set up to work in tandem with all the other sets of lights on that road. So perhaps to assure that the timings on the light aren't completely out of whack, they need to send someone out to check.

Probably, though, it's a safety feature. The power could go out because of a downed tree blocking the road, one set of lights could have been knocked out while the other still works, or maybe lightning could have struck the controller box and fried the circuitry. To prevent accidents, they probably want to either send someone out to check this or run some sort of diagnostic check remotely on the light controller.

Those are my theories, anyway.
posted by wsp at 12:03 PM on June 23

Anecdotally, yesterday we had a storm that knocked out power in various parts of the city.

I happened to be driving up a major corridor at the time. Early on, the lights were entirely dead. As time passed and the power was restored they started changing to blinking red. Another twenty blocks and everything was back to normal.

I'm thinking that blinking red (four way stop) is the default. If power is on to an individual light, without it being on to whatever widget controls the grid, it blinks.
posted by cedar at 12:10 PM on June 23

If you pull up to a red traffic light, the normal behavior is to wait for it to turn green before proceeding. The light would have to stay red quite awhile for you to finally conclude that, no, it really isn't ever going to change and you should "risk" running the red light.

Multiply this experience by a long line of irritated motorists and you see why an obviously out-of-order blinking red light is preferable to a fixed red light.
posted by SPrintF at 1:05 PM on June 23

If you pull up to a red traffic light, the normal behavior is to wait for it to turn green before proceeding. The light would have to stay red quite awhile for you to finally conclude that, no, it really isn't ever going to change and you should "risk" running the red light.

Multiply this experience by a long line of irritated motorists and you see why an obviously out-of-order blinking red light is preferable to a fixed red light.


I'm almost positive the asker isn't wondering why it's blinking vs steady. He's wondering why it's blinking vs steady for the time it would take for one cycle, then just start the traffic cycles again.
posted by Precision at 1:11 PM on June 23

Right, Precision.

Just curious why it doesn't make sure everyone is stopped, then start up properly.

Plenty of good answers here - some of which match my guesses. Nothing really definitive though. Maybe someone who knows the truth beyond the lights will come across this question yet.
posted by ChrisManley at 1:17 PM on June 23

I'm in no way sure that this utterly true, but I asked a knowledgeable friend and he suggested that mrg is closest to being right. Traffic lights are controlled by computers, timers, and sensors. If there's a power outage or some other kind of problem, they'll default to a blinking red light (treat as a four way stop sign intersection) until the Traffic Computer Nerds fix the problem. The system doesnt fix itself, so there's no automatic "reset" then business as normal. Or so my friend suggests. Take that for what its worth.
posted by elendil71 at 1:44 PM on June 23

Where I currently reside, the traffic lights are not interconnected, except in a very few cases. They blink for a while and then spontaneously restart the normal cycle after power outages.

A couple of months ago, as I was coming upon a blinking light (I don't know why it was blinking, there probably hadn't been an outage) it suddenly went into the regular cycle again. I looked around to see if there was a maintenance worker around, but there wasn't.
posted by wierdo at 2:49 PM on June 23

A couple of months ago, as I was coming upon a blinking light (I don't know why it was blinking, there probably hadn't been an outage) it suddenly went into the regular cycle again. I looked around to see if there was a maintenance worker around, but there wasn't.

They can be on timers. A lot of the lights around me switch to blinking red/yellow late at night.
posted by TheOnlyCoolTim at 3:16 PM on June 23

TheOnlyCoolTim, the lights around me aren't on timers. Besides, it was the middle of the day. ;)
posted by wierdo at 4:11 PM on June 23

It's a failsafe because the light controller has no idea whether the other lights in its intersection are functional or even exist. It would be chaos to have one direction of traffic trying to follow a normal light pattern and have another direction see no lights at all and default to the 4 way stop rule. So, it defaults to the 4 way stop until a human can verify correct operation.
posted by gjc at 4:31 PM on June 23