For the most part I don't really disagree with you, but there are ways drivers could behave unpredictably out of what might be called an excess of caution. For example, my wife just yesterday told me about something that happened to her that day. She was waiting to turn right from a side street, behind another car also turning right. The first car started their turn, she pulled up, checked that there was ample room, and so started to turn in behind them. Then the car in front slammed on their brakes, stopping partway into the lane. It seems they did this because there was a car approaching in the distance on the main street.

The thing is, that car was far enough away that both cars could have easily turned in front with room to spare. My wife had seen it before starting her turn, and realized there was plenty of time. In fact, after a few seconds, the driver in front of my wife realized just how much time they had, and finally made the turn, still before this other car arrived. Because my wife is an attentive driver this wasn't a problem, but you could certainly see a driver in her position starting to make the turn while looking to the left, and rear-ending the car that suddenly stops for no apparent reason. Of course they would be at fault for doing so, but the first driver's excess of caution, if you want to call it that, would also be a contributing factor.

Been surprised plenty by “overly cautious” drivers, mostly having to do with slowing down or stopping unexpectedly.

Folks that hit the brakes crossing a green light. Braking well before the off ramp so they are going 40 on the highway. Slowing down while merging.

The worst ones are those that are scared to go but decide too late. Like stopping in the middle of the intersection while turning left because there is a car coming from the distance but late enough they end up blocking the left lane going straight.

As a frequent pedestrian in Seattle, overly cautious/"courteous" drivers are a big nuance. Particularly drivers who are over-eager to stop at crosswalks, sometimes parking in the middle of the intersection to wait for the cross walk on the other side. Some drivers around here have a habit of stopping prematurely too, waiting for me to cross before I've even gotten to the intersection.

Sometimes it gets truly absurd. Once I was standing on the sidewalk at an intersection waiting for an Uber when a driver stopped at a green light and started honking at me, furious when I refused to cross the road.

In Portland, OR we have 4-way intersections where the stop signs alternate every 2 blocks. Drivers still tend to stop at every intersection, even when they have no stop sign.

If you try driving in Italy like you do in the UK, people will use their horn. Then you will understand.