How do you react to a situation depends a lot on what you have already decided to do. The sad thing is that most people never think about what they should do in a situation, so they end up panicking and not really deciding anything and often just going with the herd. It also doesn't help that Hollywood often shows what not to do being successful in many films for cool visual effects.
It doesn't matter if you choose fight or flight both are valid responses to critical situations, by deciding beforehand you remove the freeze that comes when you are busy trying to decide when there is no time to decide. The hesitation is not all that long but when things are happening in small fractions of a second, not having that hesitation can save your life or the life of others, as you act while others are still trying to identify the problem.
I was involved in a car crash a long time ago when a drunk driver crossed the median of the interstate. While I had imagined such an incident, not something you can really practice, I was ready to steer onto the shoulder. The only problem was that he wasn't traveling straight down the street but diagonally across it. I was able to get my foot on the brake, which amazed the cops, but violently turning the wheel at freeway speeds does nothing for a very long time. I survived and learned.
Prepare your mind to act by deciding what you will do beforehand and that way you will act quickly when the time comes, when there is no time.