I've never been diagnosed with PTSD but when I'm in high threat mode from something (an emergency, a near miss, a hostile encounter) I often find it harder at least for the next few minutes to focus on elementary things like not tripping on the stairs. If I happen to be driving when I'm in that kind of space I find I usually go into "voice-over mode" where I'm watching everything I'm doing and commenting to myself about it: "I'm in the middle of my lane and going at a reasonable speed -- speedometer says 30 -- no one next to me, two cars in front and I'm well behind them, nearest car behind me is a block away -- no traffic lights for another 3 blocks -- I'm on my way home so I continue straight till ______ street -- nobody waiting at the next crosswalk that I can see..."

As nearly as I can make out, the purpose of the dialogue is to reassure me (maybe until I'm ready to believe it for myself again) that I'm looking everywhere I need to look, seeing everything that's there to see, and making reasonable driving decisions. If instead I were to find myself weaving in and out of my lane or not daring to drive over 15 in a 30 mph zone, I'd take that as a sign to find a place to pull over and to take the easiest and/or quietest route home.

Come to think of it, I used to do the dialogue thing pretty regularly when I was a very new driver and had no idea what I would and wouldn't notice "automatically" in time to respond to it. "Pedestrian crossing sign, I'd better look for a pedestrian crossing now." I think what I most wanted to avoid was someone being able to tell me, "I told you so!"