Declaration can happen through a variety of means. Simply not accelerating would be the first one, due to friction. But even steadily accelerating on an upward slope would decrease the vehicle’s speed, and you don’t want to give vehicles behind the idea that a stop is being initiated.
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
