Just wondering if anyone here might know what would cause an entire intersection of signals to go into a blinking mode?
Today at one intersection the northbound and southbound signals were blinking yellow.
The eastbound and westbound signals were blinking red.
I mean one minute everything was working fine then all of the sudden they were stuck in blinking mode.
Weather wasn't a problem.
No electrical issues, I checked with a friend from FPL.
The control box wasn't hit by a car and neither were any of the poles.
It was the only intersection in the area that was experiencing a problem.
Unfortunately while waiting for the police to arrive and direct traffic and for the county engineers to make repairs there was a terrible collision in the ensuing chaos.
So I wonder what the hell went wrong to cause so much trouble??

Most of traffic signals runs by CPU
  There a device that called conflict monitor
  That sense all the intersection at once
   And if two interection got same green light
   It will put in flash mode. Or the CPU went
  Down can do it,  it a back up fail safe mode
 To give it a basic traffic instruction to the driver
   That intersection sound like a busy one
 With limited traffic from side street.
    It can be set up to flash red on all
   Intersection too.  Here Texas DOT
  Have them set up flash all reds.
  In marble falls texas they got
  CPU on a timer soon as it hit 10 P.M.
   It goes into flash mode.  As very little traffic at
 night US 281 got yellows all side streets got the reds
 flashing.

Almost all modern traffic signals are controlled by a electronic controller unit. Attached to this controller unit is a device called a conflict monitor or a MMU (Malfunction Management Unit). It's purpose is to monitor the status and output from controller unit and if it detects any oddities (short to ground, red and green crosstalk, etc) it'll trip the and put the intersection into flash. The typical practice here is to put the lights in 4 way flashing red so the intersection is treated as a 4 way stop. I don't think there are any signals that have one side flash yellow while the other flashes red. 

There are some locales that have the signals set to flash during hours with light traffic. Usually after evening rush or overnight. Those are usually programmed to put the intersection into flash in a more controlled way. Typically when one side turns red, it'll pause and start flashing.

The older electro-mechanical controllers don't have a conflict monitor. Those typically only flashed when manually put into flash or when triggered to by a time clock for overnight flash. They're very rare now to encounter out in the wild. You can typically tell them apart from the signature "ker-clunk" sound they made as the cams switch the lights.
   

I appreciate the interesting information about the conflict monitor.
So if the MMU puts the lights into flash mode because there is a problem with the controller would the most likely solution be to replace the controller or can it be repaired in the field??

The first thing would probably be to restart the entire cabinet which includes the controller and MMU. If it still flashes, the next would probably be to diagnose any faults in the wiring between the load switches and the signals themselves. I don't think controllers are swapped in the field that often given the complexity of programming one IMO.

When LED signals first got introduced, the low power consumption of them compared to the incandescent signals would have also tripped some MMUs. The problem was fixed in the later units.

I remember the old electro-mechanical controllers and the sound they made.  Here's one still operational in Evanston, IL when Google was by last at least.  I recall one of these developing a fault once where I believe it was both green & yellow that'd be lit simultaneously.  There are a handful of other intersections in the city with pretty old 8-8-8 or 12-8-8 signals, such as here and here, but these are presumably more modern controllers.  Also note the mix of various generations of Eagle signals and colors.  Some green, some brown, and some appear to have originally been yellow.

Here in Israel, there are traffic light systems that go flashing yellow in hours when the traffic is low.

That's common in a lot of countries, including the US. I don't recall having seen it here, though.

Flashing yellow light is a common practice to alert the drivers that the traffic lights system is malfunctioning or not working. However flashing red light is a thing that unique to the Americans.

The first thing would probably be to restart the entire cabinet which includes the controller and MMU. If it still flashes, the next would probably be to diagnose any faults in the wiring between the load switches and the signals themselves. I don't think controllers are swapped in the field that often given the complexity of programming one IMO.

When LED signals first got introduced, the low power consumption of them compared to the incandescent signals would have also tripped some MMUs. The problem was fixed in the later units.

I really appreciate all of the information that you've given me about this topic! Thank you!!

reasons for modern traffic signal controllers (CU) to go into flash:

-A problem internal in the cabinet with the serial bus

-load switch failure

-improperly programmed preemption routine violates the MMU's permissives (which signals the MMU permits to be green or yellow in relation to other greens or yellows)

-improper overlap programming in CU

-improper FYA (Flashing Yellow Arrow) programming in CU

-loose terminal connection on load bay to field wiring

-wet wiring in a splice

-data storm overwhelms CU causing CU to go into watchdog fail

-failed traffic signal in the street

-failed pedestrian signal, some bleed voltage from WALK/DONT WALK when the internal power supply fails

-car accident took down a signal pole and you didn't notice the pole is down behind the bushes

-bums try and steal the field wiring

-bums kick the cabinet causing something to pop out of place

-CU is defective and fails without external causes (modern Siemens CUs are super bad for this)

-cabinet temp gets too high and the CU shuts down.

-power surge/instability happened too frequently and either the MMU locked on a fail or the CU locked up/glitched

-poorly built intersection, poor workmanship causing shorts/opens

-neutral opens in a circuit

-a device, like CCTV or bus reader, sharing power with the cabinet has failed

-slugs crawled up onto the 120V field terminals and caused a "dual indication" fault in the MMU

-ants decide they like to nest inside the MMU and cause the MMU CPU to fail (for some reason, ants like MMUs and ethernet switches)

-a signal timing engineer remotely downloaded erroneous dial card


BTW, replacing a modern CU is getting easier. Most of ours support a "data key" and certain firmware versions also allow us the option of using a USB stick to transfer data. We also have access to the central control system on our laptops, and if we need to we can connect a replacement CU to the network and program it with the data from the failed CU.

@takemorepills: MASSIVE THANKS for that amount of interesting information!!
You mentioned a problem with the serial bus inside the cabinet or a load switch failure, are these something a crew can routinely repair??
Also when a crew first arrives at a flashing intersection, what would be the first thing they have to do??
Shut the system down and perform various diagnostic checks??
Also could a faulty splice box that a signal head hangs from cause the MMU to act??

@takemorepills: MASSIVE THANKS for that amount of interesting information!!
You mentioned a problem with the serial bus inside the cabinet or a load switch failure, are these something a crew can routinely repair??
Also when a crew first arrives at a flashing intersection, what would be the first thing they have to do??
Shut the system down and perform various diagnostic checks??
Also could a faulty splice box that a signal head hangs from cause the MMU to act??

1. Traffic Signal Electricians  (TSE) have a wide range of competency. Some people understand how the serial bus system works, most people it's a foggy mystery. What usually happens though, is a serial connector comes lose, or a CU shuts down unexpectedly and of course the serial bus idles and the MMU sees that as a logged reason to go into flash. Rarely, a BIU or SIU might actually have an issue.

2. Most TSEs randomly replace load switches. Then hope for the best. TSEs who are more thorough, will check to see if a LS is bad, not assume.

3. Many TSEs will just hit "reset" and hope the signal begins running again. Personally, I will diagnose the signal. The MMU is like a 48 channel DMM(voltmeter) and is looking at every wire going out to the field signals. It records the values at time of flash, and you can look at each individual voltage. Also, the MMU will tell you why it went to flash, "dual indication" "serial fail" "red fail" "CVM" "conflict" and in some cases it will even tell you what the serial data said the cabinet should be doing and compare it to the fault the MMU "saw". For example, you may find the MMU in flash, and the display says "dual indication" and if you scroll down on the LCD for more info, it will saying something like "The CU drive for CH12 was GRN, but the LS output was GRN and RED" that indicates that the load switch was putting too much AC voltage onto the RED circuit, and the MMU confirmed the CU was actually only driving the GRN circuit. This is common when a wire breaks somewhere or all reds burn out, the load switch will "float" because it's unloaded AC output can't turn off fully without a load present.

4. Yes, also for span-hung signals using solid wire (my city thought this was a good idea) the constant movement can break the copper conductor, with no apparent external evidence, at this often happens at a signal entrance head.

1. Traffic Signal Electricians  (TSE) have a wide range of competency. Some people understand how the serial bus system works, most people it's a foggy mystery. What usually happens though, is a serial connector comes lose, or a CU shuts down unexpectedly and of course the serial bus idles and the MMU sees that as a logged reason to go into flash. Rarely, a BIU or SIU might actually have an issue.

2. Most TSEs randomly replace load switches. Then hope for the best. TSEs who are more thorough, will check to see if a LS is bad, not assume.

3. Many TSEs will just hit "reset" and hope the signal begins running again. Personally, I will diagnose the signal. The MMU is like a 48 channel DMM(voltmeter) and is looking at every wire going out to the field signals. It records the values at time of flash, and you can look at each individual voltage. Also, the MMU will tell you why it went to flash, "dual indication" "serial fail" "red fail" "CVM" "conflict" and in some cases it will even tell you what the serial data said the cabinet should be doing and compare it to the fault the MMU "saw". For example, you may find the MMU in flash, and the display says "dual indication" and if you scroll down on the LCD for more info, it will saying something like "The CU drive for CH12 was GRN, but the LS output was GRN and RED" that indicates that the load switch was putting too much AC voltage onto the RED circuit, and the MMU confirmed the CU was actually only driving the GRN circuit. This is common when a wire breaks somewhere or all reds burn out, the load switch will "float" because it's unloaded AC output can't turn off fully without a load present.

4. Yes, also for span-hung signals using solid wire (my city thought this was a good idea) the constant movement can break the copper conductor, with no apparent external evidence, at this often happens at a signal entrance head.

The information you give just keeps getting better and more fascinating!
While I may be stepping over the line here with the questions but is there an easy way when you arrive at a faulty intersection to confirm that the CU is bad and that's the reason for the problem??
Also have you ever seen a signal head displaying more than 1 color, say a green and a yellow, I'm certain that I may have seen that once but I guess with the MMU it's just not possible.
Anyway thanks for your time takemorepills, this is why I joined LG to learn.

is there an easy way when you arrive at a faulty intersection to confirm that the CU is bad and that's the reason for the problem??

Also have you ever seen a signal head displaying more than 1 color, say a green and a yellow, I'm certain that I may have seen that once but I guess with the MMU it's just not possible.

CU: The older CUs rarely went bad. We have menu-driven CUs like Eagle M34 and TCT LMD8000/LMD9200 that have been in the field, operating reliably for a quarter century! However, we'll remove an M34 or M52 and replace it with an M60, and those M60's are just horrible. They have so many hardware issues, fortunately though it's pretty easy to see if the CU has failed, many times the CU is just dead (internal power supply failure) or, it has a log of OFF/ON/OFF/ON/OFF/ON too many times, too frequently, and you can actually check the MMU logs to see that cabinet power never cycled, it's the CU power supply failing. The modern M60 CU rarely can make it 18 months in the field, usually it's not uncommon to see them come back for warranty in 6 months or less.

I have seen a traffic signal display multiple colors before, usually it's the entire intersection. On older NEMA TS1 cabinets, they had a main bus relay that used mercury, and when the Conflict Monitor (older version of MMU) would go to flash, the mercury contactor would stick "closed", so you'd have red or yellow flashing, while the signal was still displaying the other 2 colors as the CU would be stop timed on whatever color it was on, so we'd arrive and the reds would be flashing but the greens would be on also. Newer cabinets have an additional relay to de-energize the 24VDC to the load switches to prevent that from occurring.

The most interesting one I've ever seen was R/Y/G all being displayed at once. It was in an older NEMA TS-1 cabinet, so it used a CM, not an MMU. The internal clock of the CU had gone bonkers, causing the CU to run through R/Y/G so fast every cycle that it just looked like all 3 colors on at same time. Those CMs do not have a minimum time function, the MMUs do have a minimum 3 sec of YEL function, so if the cabinet had an MMU it would have been in flash instead. It was pretty cool, the LCD screen on the CU also showed the time running wickedly fast, and all 3 intervals displayed on the LCD looked like they were all stuck on at once. Mind you, they were never stuck on at once, a CM would catch that.