Automotive engineer here! I don’t do lights, so I have no idea!

BUT… I kinda do. You want the lowest PWM frequency you can get away with. In this case, at the back of the car, furthest from the battery, you really don’t want a 8kHz PWM nor do you need it. It costs money to isolate the supply demand, so you don’t want a noisy field for no reason. The “good enough” frame rate is 60Hz static, not moving, no other flashing lights, not using a camera, etc.

60Hz or 60fps has issues though. If you expose a PWM LED to another flashing light or movement you get really bad imaging. Imagine you took an LED in your hand and shook it, at 60 Hz you will see snapshots of where the LED was as you’re shaking it. At 240Hz you will see a blur. Guess which is better for a vehicle?

I figure most car LED taillights internal to their housing would be 200-1000Hz depending on factors but I haven’t ever measured.

200Hz PWM is a really common value. No need for Samaritan base-12 here.

For halogen and incandescent, we use PWM, fun fact. Low Hz though! About 88Hz, depending on voltage. You might wonder why. We can get 98% of the light output with 85% of the required wire. It’s all about weight and cost. Although not many vehicles use this anymore.

Imagine your eye's effective integration time (the period over which it "samples" light) shrinks during a rapid saccade—say down to about 5–10 ms. Under steady conditions, our flicker fusion threshold is around 60 Hz, but that’s because our retina integrates over a longer window.

If we want to “smooth out” the PWM cycles so we don’t see discrete pulses, we need a few cycles (say, 3–5) within that 5–10 ms window. In other words:

    For a 10 ms integration window:
        3 cycles → f≥30.01 s=300 Hzf≥0.01s3 =300Hz
        5 cycles → f≥50.01 s=500 Hzf≥0.01s5 =500Hz
    For a 5 ms window:
        3 cycles → f≥30.005 s=600 Hzf≥0.005s3 =600Hz
        5 cycles → f≥50.005 s=1000 Hzf≥0.005s5 =1000Hz

So, to cover worst-case scenarios (rapid eye movement, bright conditions where the eye’s temporal resolution is higher), the PWM systems for LED lights should be rather in the ballpark of 300–1000 Hz than 200 Hz. Given that one would be viewing headlights some 2 meters away (impacts the what is the view angle change and speed).

And yes, what we are now seeing in cars is super annoying. Similar disregardful to user comfort can be seen with HUD displays (at least with Volvo).

Fascinating! I've played with VW's BCM stuff, but for some reason everything except the lighting is translated into English on the common factory equivalent scan tools, so it's a matter of sorting though a bunch of "lichtfunktion" this and "dimmwert" that.

> We can get 98% of the light output with 85% of the required wire.

I guess a 12v bulb can survive a much higher voltage than it's rated for (which needs fewer amps for the same watts, so thinner wire) if it's pulsed, like 120v assuming 10% duty cycle, as long as the frequency is dialed in such that the filament is just about ramped up to its rated temperature when power is cut. Very clever!

Actually, you want to use a DC-to-DC converter that properly delivers the constant desired reduced voltage, rather than making a ridiculous stroboscopic light (aka PWM).

Constant current is more expensive and does not scale to a lot of LEDs or high power LEDs well. The scale wouldn’t be an issue with the tail lights, but the power kind of is. Take a look at modern taillights sometime, they all have heatsinks on them. I’m sure there are some; but I am not aware of any constant current taillights.

> For halogen and incandescent, we use PWM,

Must be fancy cars then. All I've ever owned used relays to switch the lights, just straight 12v onto it no pwm whatsoever.

That used to be the case, of course.

We PWM those now because the PWM drivers are there anyhow, and with a filament, it’s still lit and bright during the off pulses. It’s cooling, but you really can’t detect it.

> introduced by PWM dimming, but why would that be a low enough frequency to bother people?

The human fovea has a much lower effective refresh rate than your peripheral vision. So you might notice the flickering of tail lights (and daytime running lights) seen out of the corner of your eye even though you can't notice when looking directly at them.

For sure. I summarized my particular sensitivity too aggressively earlier, but I tend to see flicker straight on up to 65 Hz and peripherally up to 120 Hz if it's particularly egregious (i.e., long valleys) but usually up to something less. In any case, I've never noticed car tail lights flickering even peripherally, despite video artifacts revealing that they do flicker.