Some EVs and hybrids have a 'one foot driving' mode where if you take your foot off the gas, it does start to actively brake and you will eventually come to a complete stop. Technology Connections did a video on it a little while back, showing how it can be bad if the brake lights aren't programmed to come on in this situation.

You’ve spelled out the entire problem but have come to incorrect conclusions. If you’re directly behind a car, it becomes hard to sense acceleration or deceleration of other vehicles. More importantly, the brake is often used to decelerate on highways and not initiate a stop. This all leads to a lot of confusion and ultimately crashes every year.

The situations you mentioned are exactly the problem areas. Not accelerating it slowing down, often quite quickly, to make an exit. Dangerous. Going up a slope may or may not change speed, which needs to be known. Again dangerous. Same with downhill which is actually more dangerous not knowing how hard braking is happening.

So we have the technology to fix this. We can invent either variable break lights that change brightness or zoned lights that tell you “I’m slowing down” versus “I’m braking hard”. We could fix this because cars are dangerous enough as is

A rant? Alec? Surely, you jest.

I am honestly flabbergasted that people think this is a good idea. This would basically mean that the brake light would stay on almost all the time that you weren't actively accelerating.

There would be so many more highway accidents. I heavily rely on the brake lights of the cars in front of me to drive defensively. They tell me when I should expect to put my brake on.

If brake lights turn on just because some took their foot off the gas, I can't tell the difference between a gradual slowdown and an intent to stop, so it would make it way more likely that I wouldn't brake in time to avoid a collision in a sudden stop of traffic.

Brake lights should only ever indicate that a driver has their foot on the brake. I absolutely must have this information delivered to me reliably. If the meaning of brake lights changed as recommended here, I would be legitimately scared of highway driving.

Engine or regenerative braking can very quickly slow down a vehicle but may not activate the brake lights depending on the manufacturer.

Or crashing, I guess.

EVs have regenerative braking, where when you let off the accelerator, it immediately starts slowing down, quickly. But the brake lights don't come on. This would make driving behind EVs safer.

I would like to share this with you: https://youtu.be/U0YW7x9U5TQ

Which is also why it doesn't come on if you're motor breaking" (is that the right term?)

I believe the term you technically want is engine braking

Because there are laws that specify when the brake light has to come on, and it isn’t when the car shows down (slightly).

To be clear, the laws say when it must illuminate. They do not (in the US) prevent illuminating it for other reasons in any way. The law says the light must illuminate/burn if you are actively pressing the brake pedal, but does not prevent it from also illuminating if a certain amount of regenerative braking is applied or a deceleration is detected. Theoretically an automaker could get away with making the brake lights simply always illuminate (and that loophole would be fixed in days, so no one does it).

The problem is with electric cars that can be driven with one pedal most of the time

Some newer higher end models do this. It's an emergency brake feature that flashes the brake lights rapidly when braking heavily.

Should be standardized or something similar should be

The brake light means “pay attention to this car”.

Determining how much deceleration it’s experiencing is for the driver’s depth perception.

So instead of the message being “I’m slowing down by X amount here, so match that”,

the message is “I’m braking, so initiate the procedure by which you determine then match my X deceleration”

This does exist on certain vehicles, like ambulances.

But I've heard (unconfirmed) that it would be illegal in several states to put on passenger vehicles.

Between XKCD and Alec, the whole of human knowledge's pretty much covered.

More accurately, if they added one single extra component like that, they would raise the MSRP by like $500. Because we live in capitalist hell. Therefore, it's included in higher quality cars, and/or as an optional feature. Like most safety anything, they won't automatically include it in anything unless they are literally forced to by law. And even then though sometimes not and say they did. And still up charge you for it. Isn't capitalism fan-fucking-tastic?

I beleive that should be common practice when down shifting to slow because it's not much different from braking without any brake lights. However, I've come to also trigger the lights even when coasting down in top gear. The slightly harder engine braking in a manual, even in top gear, combined with the increase of distractions, has me concerned about the same thing as you.

I heal-toe, so I'm pressing the brake as I'm engine braking. But I think I've read somewhere that newer manuals actually will turn on the brake light on heavy deceleration via engine brakes, I just don't know firsthand because every very I've owned has been old as shit. Ditto with electric cars that have aggressive regen braking.

Activating the hazards when hard breaking is also just common practice to do manually (at least in western Europe)

I reach for the hazard lights any time I'm about to break hard or if someone in front has done something that will make me slow unexpectedly.