Be predictable, not polite
When you’re driving a car, the best thing you can do is to be predictable with your actions. If others know what you plan to do next, the odds of staying safe remain quite high. Conversely, if you do unpredictable things, even with good intentions, things can go wrong quickly.
As a quick example, see the car in this gif stop suddenly to try to let someone merge in, nearly causing a major accident behind them: