Flashing lights

This is something that has bugged me forever and what gets me is that lots of people have no clue WTF I am talking about.

Some lights flash at high frequencies. I find many of them distracting and annoying. This is especially the case in a car at night. The worst are the yellow lights on traffic cones at road works. I don't mean the blinking a couple of times a second type flashing, I mean that the yellow light is actually strobed, probably at around 1KHz... But the new solar powered cats-eyes are strobed too, and many newer cars have pulsed red LED tail lights.

When not moving and looking fixed at one of these lights they look like they are on solidly (if they are much more than around 50Hz). But you don't look in one fixed place. When moving my vision from one object to another, or when in a car and the lights are moving, they leave dotted patterns on my peripheral vision and that drives me mad. They cry our for attention. I find myself looking to the flashing light. Its distracting and annoying and possibly dangerous.

Some lights are not as much of an issue, and in fact I think it is the on/off ratio that is what causes the annoyance. Normal yellow sodium street lights as you get on motorways are probably on for 90% of the time or more and around 100Hz I would guess. They do leave a dotted line on my vision when looking from one place to another, but it is almost solid and the edges are not as sharp. The traffic cone lights however are proper strobed, maybe 1% on and 99% off or something. Even though they may be up near 1KHz, they are probably the most annoying.

Now, there are clearly very different ways people see things. I have tried at great length to explain this to friends and family when in a car with them, and they just can't see it. They swear blind that the lights are on solidly. I have so far only found one person that can see the issue (and they find it annoying too).

Do tell me I am not totally mad!


  1. I haven't noticed the traffic cone lights, but I *have* noticed some really weird strobe-y effects in my peripheral vision from those cats eyes.

    It was bloody distracting, because I couldn't work out how cats eyes could possibly be doing that, and probably paid more attention to them that I perhaps should have done...

  2. Not all of the traffic cones are like that, but I think it is more and more common. Some times you have a row of them and some are solid lights and some are strobed.

  3. Well you can count me as another one who sees them and finds them annoying - car rear lights especially because they are getting more common, and you see them everywhere, not just at road works etc. I see a line of bright dots or short stripes when I shift my gaze (to/from the mirrors, for example).
    I'm the same with CRT displays (remember them? :-) I couldn't stand having a frame rate less than about 70Hz - less than that and they strobed in my perpipheral vision which was incredibly annoying, but a lot of people didn't know what I was talking about.
    I think what car rear lights are doing is using LEDs with Pulse Code Modulation (PCM) to control the brightness to use one set of LEDs for multiple functions - low On:Off time ratio for rear lights, high On:Off time (maybe On 100%) for brake lights. It's a clever engineering solution, but I think they need the frequency to be a lot higher.
    Start a campaign: Drivers Against Strobing Light Annoyance (DASLA)? :)

  4. Yes, that is what they do, and typically solid on for break lights. You can see how a designer than cannot see the same way would think it clever!

    What concerns me is that I think the cone lights are fast. Using a camera and comparing the dot trails with street lights I worked out around 1KHz. So I am not sure how fast PCM LEDs would have to be to not be annoying.

    Anyway, glad I am not going mad. I did wonder whether a flashing red rear light on cares was actually legal. There are laws on flashing and steady lights of various colours on cars.

  5. I notices the cats eyes some time ago, but I thought they were reflective, so how do they flash (maybe I'm missing something here!).
    The car rear lights are the worst for me, they drive me totally mad. Especially when sitting at traffic lights, and they are right in front of your face!

  6. It is new cats-eyes which are solar charged and then light at night, but are strobed to give a brighter light effect for less power. I don't think they are too common yet, but they light up the road much further. If they were not so distracting they would be a great idea.

  7. No you're not totally mad. I see all of those things and I hate it - like it's frying the inside of my eyes. I started noticing it on some of the crappier LED traffic lights and people's rear lights a few years ago.

    I'm sensitive to CRTs at less than 75Hz refresh rate which has historically been a pain where the setting can't be upped. I am also sensitive to flickering of fluorescent lights which many others don't notice. I walk out of rooms and shops with flickery lights and refuse to use low refresh rate CRTs.

  8. I think it's something to do with what clock speed your brasin works at.

    Personally, I didn't even realise that pelican lights flashed 'till someone explained what 2Hz meant.

    Isn't POV *GREAT*?

  9. Can't say that I've noticed the rear car lights or the cone lights, but the cats eyes are incredibly annoying.

  10. I've seen headlights that look like they're flashing and they're really annoying if you're in front of one. That's some kind of LED system too I believe.

    Never noticed any of the others. I notice more that a significant number of cars around here only have one (and sometimes no) rear/brake lights.

  11. If you're ever over in East Anglia, drive up the A11 towards the A14 (from Cambridge to Newmarket) the streatch of the A11 between the last junction and it merging with the A14, they test these cats eyes, they're insanely annoying and on that road they test others too, some of them are useless.

  12. Hi, I have the same "symptom" that I also see trails behind LED car rear lights. This is quite annoying. I have discussed it with some friends and most of them don't even understand what I am trying to explain.

    Hopefully car manufacturers would change the PWM controlling of LED car rear lights so that the trail would go away.

  13. Is this all down to the fact that by pulsing the LED not only do you make them use less power but you can also crank up by some serious margin the intensity of the LED.

    I had a chat about this some months ago with a friend and he just looked at me like I was bloody stupid.

  14. Hi, I have studied this topic a bit more. Would this be related to Palinopsia?

  15. Never heard of that before. Reading wikipedia on the matter I don't think so...

  16. Please see Google image search on "palinopsia". From there, there is a link to page: http://www.migraine-aura.org/content/e27891/e27265/e45446/e57090/index_en.html

    On that page, one person describes how she sees "polyopic trail" (I don't know what polyopic means) behind moving objects. This is what I see behind some newer car backlights, specially newer Volvos.

    I'm just trying to find out what this is about as this is very annoying "feature" of seeing flashing trails between LED backlights.

  17. Actually, there has now been a publication about what seems to be exactly this! JE Roberts and Arnold Wilkins ... :-


  18. Thank you very much for this reference! It describes exactly what I'm seeing :-)

  19. Thanks Simon for this refeference! It describes exactly what I'm experiencing.

  20. Traffic one lights are also a big problem for me together with many LED tail lights that jump about. Volvo seem among the worst for me. Commercial vehicles are using large LED clusters that are awful. The red and green men on pedestrian crossings go wild at night. I have problems with DLP projectors, especially the early ones that gave me the worst headache in my life. I think the motor industry needs to review these lights and also the excessive brightness at night. Why can;t they incorporate a dimmer that works when the headlight turn on. It's well known that they cause dazzle and reduce visibility of poorly illuminated objects like idiot pedestrians and cyclist who wear all black clothing at night.

  21. I have been trying to get to the bottom of this for two years. First experienced driving towards a large garage forecourt price per litre sign. Saw a whole row of identical copies of the illuminated price, maybe 8 or more, strung out in an equally spaced line across the night sky like data appears in a head up display. Certain garage signs now always trigger this for me in low light. Then started seeing multiple copies (20 or so in a row) of the redman greenman at some pedestrian crossings; then some car lights leaving trails of lights or multiple copies. Then the cats eyes in the Hindhead Tunnel started sweeping across the width of the road in multiple sets. I noticed this was associated with head or eye movement and it is present if either eye is closed or both open I have presumed it was due to some kind of damage in the visual areas of the brain.


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