Cars are not animals, they don't have subtle body movements that you could read. If you can see what the car is doing, it is already too late. The idea of turn signals is that you let other drivers know in advance. Even if they are "stupid" and can't "read" your car. Even if you are stupid and you are doing something really stupid, signals give people around you time to notice you and avoid you. We drive to get from point A to point B, and different rules and mechanisms are there to make the whole process safer. Go feel superiority somewhere else.
In the majority of cases you can anticipate what other drivers are doing based of subtle changes in speed or lane position. If someone is slowing and moving further to one side of the lane than they have been, it's a pretty good indication they are preparing to turn.
It would be nice if people could explicitly signal that by using their indicators, but most people are fairly predictable, and the subtle movements are there if you watch for long enough.
By majority it's 100%, physics exists and the time required to maneuver in being capable of turning or lane changing is plenty to clearly signal what it will do. The reality is that if you need turn signals you are bad at driving and should not drive. Most humans are not capable and cars should not be how the majority of people get around.