Unless you instantly kill every person you see, I don't see how you can save yourself from someone intent on killing you.
-Guy walks by your vehicle, you don't shoot him. As he gets next to you, BANG, he shoots you by surprise.
-Guy on a hill sniping people, you catch a round before you even know he exists.
-Guy drives past your vehicle going other way, as he passes your window, he pulls up his gun and shoots you in the face.
There is nothing you can do about it if some random person waits until the last second and pulls a hidden gun with intent to kill you. That's the scary part because we can't do anything about it.