Excellent *quality idea, but is it legal?

Boosting this quality contribution up in the Hot Listing - declared quality by newtboy.

Most states have laws against lights in this color flashing if they are not OEM or on an emergency vehicle.

Exactly !
Driving ont the freeway @60Mph and having the car infront break it would invaluable to know if he was just slightly slowing or it was an emergency break.
I'm sure there must be some reason this hasn't been implemented....

it should also be brighter / bigger the slower the car is moving

This is a decent idea:

https://www.youtube.com/watch?v=OIBwWGh9I90

FYI, there are a number of high-end cars that have this built-in. BMW has different versions across a lot of their cars, and calls it the 'Brake Force Display'.

BMW found it was less distracting and more intuitive for other drivers if the brake force was indicated by a few stages of big lights going on (3 or 4), with the last stages also having one or all of the brake lights slowly flash in the 'hazard light' mode if you really slammed the brakes.

Motocyclists also commonly buy aftermarket flasher modules that indicate when they are braking heavily, which you can find on amazon.

Proximity sensor? Why not just a simple potentiometer?

Yeah I don't think this is a new invention by any means. I swear I remember reading about this type of brake indicator quite a few years ago.

Comment hidden because you are ignoring dag.(show it anyway)

I remember seeing the hazard lights flashing for hard braking in Erurope. Not sure if it's legal in the US or other regions as i've never seen it.

I'm gonna jump on the "always wondered why this wasn't a thing" bandwagon too.

It seems like such an obvious and cheap idea, there must be a reason why it's not implemented.

I wonder if it's one of those things that sounds great in theory but has some terrible side effect in practice (e.g. maybe no-one pays attention to a short brake tap? I dunno)