# eli5 why at stop lights turn signal flashes sync up for only a little bit

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eli5 why at stop lights turn signal flashes sync up for only a little bit

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I assume you mean turn signals on different cars!?

Because the blinking rate of turn signals isn’t regulated to the same precision as, let’s say, a wrist watch needs in order to be useful. Different cars will have slightly different frequency (blinking rate). This means that some cars will blink faster than others, resulting in the flashes syncing up, going ahead and then catching the slower ones again, effectively “lapping” the slower turn signals.

In any two or more consistently repeating sets of patterns, there will always be a moment in time when they appear to “synch up”, because sooner or later the patterns are on or off at the same moment. So for example of you have a flasher that flashes on every 3 seconds, and another one that flashes every 2 seconds, they will flash at the same time every 6 seconds, 12 seconds, 18 seconds etc. etc.

This is a tough one to explain simply but ill give it a try:

The short answer is, they don’t sync up. At least not the way you think they do. What you feel to be synced up is in fact just them being close enough together for you not to notice.

An example could be done with 2 lights flashing at very close but slightly different times, this makes the time difference between blinks longer. By making the blinks really slow, it would be easier to spot this fake syncronizing:

Light 1 takes 59 seconds to flash
Light 2 takes 61 seconds to flash

If they start at the exact same time, after 59 seconds the first will flash and 2 seconds later the second one will flash, at this point they are already unsynced. Then once 1 minute 58 seconds have passed, the first light will flash again, with the second one following 4 seconds behind.

This repeats until 3,599 minutes have passed, at which point there will be 1 more perfectly syncronized flash.

So the main reason that car blinkers seem to sync and desync is because they have similar, but not identical pauses between blinks, us humans are just incapable of seeing the difference between 0.995 seconds and 1.005 seconds.

The turn signals are controlled by an electrical relay. Basically, an automated switch that turns the power on and off at a designated interval. The interval is set by the gov’t and probably has a timing from x.x to x.x seconds. Lots of different manufacturers make these relays and sell them to the auto manufacturers. The relay from Company A will work at a specific interval where the relay from Company B will have a slightly different timing (either slightly faster or slower).
Since we’re talking about milliseconds difference, the timing will eventually sync up for a few flashes. Basically, the faster turn signal will catch up with the slower signal, sync up, then pass it by and be out of sync again.

Because they are not synchronized, they are just really close and kinda blink together until the difference in flash speed becomes noticeable

The reason they are almost never synchronized is that the speed of turn signals doesn’t need to be very precise, and even if your car is modern enough to have computer-timed blinkers, every manufacturer sets their own speed sometimes different for every model, so it’s unlikely you find the same car in front of you. But it is definitely possible, i had my turn signals synchronized with the car in front for several minutes one time

As a side note, whenever you see a car with a turn signal that’s blinking really fast, that’s the relay telling the owner that a bulb has burned out.

[An interesting video if you have the time](https://youtu.be/2z5A-COlDPk) “Technology Connections”