I had three emergencies come up while I was in my twenties. One was the famous tree incident (I was inside a tent when I heard warning shouts, and when I looked out I saw an 80-foot aspen in mid-fall heading straight for my face), one was when I was meeting a friend at a restaurant and had a guy follow me down a deserted street shouting and hassling me, and one was when I was sailing a 1-man boat on a small but cold lake when the sail came loose and the rudder fell off -- apparently whoever rigged the boat (not me) had cut a couple of corners.

Of the three, only the first was life-threatening (the guy was just being a jerk), but I reacted pretty much identically in all three situations: emotions on hold, slight feeling of unreality (shock?) and brain in overdrive until the situation was dealt with. THEN -- especially in the case of the tree -- I freaked.

Not so sure about driving emergencies, though. I've had a couple of minor ones, and I have the impression that I respond a little less collectedly to them.