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.

What if you don't have 2 eyes?

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.

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).

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 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.

Actually, a least some cars (probably more than you think)/have some sort of "emergency" breaking signal, which often result in the usual breaking signal blinking for a while. You need to seriously slam the brakes to see it though. E.g: https://m.youtube.com/watch?v=j_osu1WgGMU

There is also a nice technology connection video talking about braking lights for electric cars and in general, which can be dangerous.

I know cats that blink the middle light if you brake hard.

EDIT: I'm not fixing that typo

Most modern cars now put the hazards on automatically when the driver brakes hard.

I seem to remember that when BMW first rolled out LED taillights they did something like this — they were brighter when the driver absolutely slammed on the brakes. No idea if that was a concept or in production cars though.

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

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”

https://youtu.be/U0YW7x9U5TQ