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When do my brake lights come on?

My daily commute has me on a Southern California toll road with a fairly large hill in the middle of it. Traffic, as is usual in SoCal, often has us driving well over the speed limit (say, 75-80 in the posted 65) and somewhat close to each other, maybe 2 seconds or less distance between cars. Ehhh, probably mostly dangerous, but somehow SoCal drivers manage to not kill each other all the time.

What I like about my Tesla is that the regen braking is so good and strong, I rarely have to use the real brakes. But I noticed that there is a big gap between when regen braking starts slowing you down, and when the brake lights actually turn on - warning the drivers behind me that I'm slowing down. With the 2 second gap, it's even more important that the brakes light up, but if I take my foot fully off the gas to tap the brake, my car will go into full regeneration braking PLUS physical braking. If I only use light regen braking, I can't be sure if the brake lights are on or off. Sometimes I look at the little icon of my car in the Instrument Cluster, but it's hard to always see if the brake lights are on or off - and I don't want to take my eyes off the road for that long, traveling that fast, so close to other cars.

Is there some actual trigger/level of regen braking that makes the brake lights turn on, and can it be adjusted?
...can it be adjusted?

No. It's done by Tesla and not user-configurable

...Is there some actual trigger/level of regen braking that makes the brake lights turn on...

Yes. But the parameters are unknown.

Since the early days in 2012, the brake lights were really synchronized to the regen. Every little regen would trigger brake lights.

The problem was: Motorcyclists complaining that Tesla consistently did a brake checking with motorcycles which could pose a danger if they overreacted and also applied brakes and conducted emergency maneuvers for a non-event.

Tailgaiting trucks and cars also complained but it's less of a danger because they don't lose balance the same way as motorcyclists do.

Thus, Tesla then quietly adjusted the brake lights so that they only light up if there's a worthy braking event from the regen that would slow down the car to warn others.
Ahhh that makes sense. Now I also think my use case is unique in that as we're all traveling down hill one assumes that we should be accelerating in the absence of brake lights - even if you're coasting, you would be accelerating because the road is all downhill. Except for my Tesla. Under moderate regen, my car is moving at a steady speed, because regen keeps me from accelerating. But, because it's not deemed a worthy braking event, the brake lights don't light up, and the cars behind me sneak up on me so fast and get confused.

On flat land or at least in a place where everyone else isn't madly speeding down the hill, I guess the issue is less pronounced.

Thanks for the insight!
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I think it's at a certain level of kw/hr regen levels. If you are only regening energy at 1-5 kw/hr it isnt enough to slow your car down very fast (much like taking your foot off the accelerator in a normal car) but at 25+ kw/hr regen you slow down a good amount so that someone behind you would have to put on the brakes. So my gut thinks there is some threshold at each MPH that turns on your brakes. something like
0-20 mph : greater than 5 kw/hr regen then turn on the brake lights
20+ mph : greater than 15 kw/hr regen then turn on the brake lights

Just a guess though, but it feels like it is something similar to that.
Agree it feels like G-force activated. If you have space, (which the driving conditions posted-do not,) you can manipulate the go-pedal to barely slow down without brake lights. It works great, just not suited that well for close, high speed, dangerous driving.
Your dash display will show you when your brake lights illuminate. Depending on your situation, sometimes the lights will come on and others not so much. It is not determined by brake peddle application, but on negative G forces.

Using regeneration on a steep downhill should not illuminate your lights unless it is enough to actually slow your speed down.
This is a good question. One-pedal driving is great but it creates new challenges. I believe drivers need to know when the brake lights come on for safety reasons when they come on as a side-effect of regen. The only way to know if the brake lights have come on at this point is to glance at the car graphic in the binnacle. With recent software updates, the graphic is a decent size on a two-lane road but on the highway, it shrinks down to a very small size making it difficult to see the brake lights when they turn on.

What I've been doing is watching the graphic while using regen and experimenting with how much regen is required to turn on the brake lights (at least when the graphic is big enough that I can see the lights). If someone is behind me, I'll use more regen briefly to turn on the brake lights for a moment to indicate I'm slowing down. Over time, I've begun to learn how much regen is needed to turn on the brake lights. What I learned is the way I had been using regen in a more gradual manner does not normally cause the brake lights to come on which can be a safety issue since a car behind me may not see any brake lights at all until I've almost come to a complete stop.

Tesla needs to update the software so there's a better visual indicator for the driver to show when the brake lights come. This should be some indicator that supplements the brake lights on the graphic since the latter varies so much in size and the brake lights may not be visible enough.