Before
Like you say, it's easy to judge reactions when you're watching a recording but when you're actually driving you might be focusing on something else like a sign up ahead. And that really affects your reaction time.
The real mistake here is trying to undertake someone. That's always a risky manoeuvre for this exact reason. People don't expect someone to pass them on the outside lane. A good driver *should* always check their mirrors in case someone *does* try to undertake you. But let's face it, most drivers aren't very good.
I'm not blaming OP. The person in front shouldn't have turned by crossing the lane. That was insanely stupid. But OP could have avoided this accident by not attempting a risky manoeuvre.
After
Like you said, it's easy to gauge the reaction when watching a recording, but if you're a real conductor, you can focus on something else, like pre-registering. And it really affects your reaction time.
The real mistake here is trying to make any of them. Therefore, it is always a risky move. Unexpectedly, people had to pass them in the street outside. A good driver should*always check his mirror*if someone is trying to catch you. But let's face it, most pilots aren't very good.
I don't blame the OP. The person in front must not turn when crossing the line. This is really crazy. But OP can avoid this accident by not trying to maneuver ahead of risk.