Well I don't know the frequency, but it is easily noticeable to me on most modern cars when moving my eyes. It's definitely not a question of aftermarket modules.

The PWM I use at home on an ESP32 is supposed to be in the 1-10 kHz range and I don't see flicker when moving my eyes.

Yeah, you'd be superman if you thought you could see a khz flicker.

I associate flickering headlights with aftermarket because cars with halogen headlights use low-frequency PWM to regulate down high system voltages to 12V. So if you install aftermarket LED kits on those cars you are going to see a PWM of ~90Hz which is awful.

> use low-frequency PWM to regulate down high system voltages to 12V

That can't be right. Cars are primarily 12VDC.

System voltage when the engine is running are higher otherwise the 12V battery would never charge. Can be as high as 18V. So cars with halogen lights with just chop it down to 12V.

Here's a random video I found demonstrating the phenomenon. https://www.youtube.com/watch?v=82tFav8oGFI

Interesting, so they're using PWM to hold the headlights at constant brightness when the alternator runs. Wiring a capacitor in parallel with the headlights should fix that.

Actually I forgot to mention that is it mostly the case for taillights (the most noticeable since you're going to follow cars quite often) and accent lights (like the vertical strips on the front of recent Peugeot cars).

Not really on headlights I guess so I might be off-topic. And although I understand how some people can find it annoying, I don't really mind them. But I can definitely see the flicker and can compare the duty cycle between different cars.

I am lost why anybody would even want them.

The usual ~200kHz PWM is cheaper, more reliable, smaller, and usually more powerful than anything you can build on the ~100Hz. Besides everybody sells chips for ~200kHz, but finding anything that can operate at ~100Hz is a challenge.

I can understand cheap household ones using the mains frequency to avoid an AC/DC conversion. But AFAIK, using them on a car is stupid from every conceivable point of view.