On my first CK session day at Stanford, my wife accompanied me because I wasn't sure how I would be feeling afterwards. As it turned out, I drove both ways because I felt no change after being zapped. On the remaining two sessions I went alone and had no problems driving. However, my drive time was only a little more than an hour each way. Dr Chang is very receptive to questions via email so it may be a good idea to email him and ask what he thinks. Dr Chang's email address is on Stanford's website (stanfordmedicine.org).
My first session was at 11:00 AM, my second session was at 11:30 AM, and my last session was at 9:00 AM. They would like you to be there 30 minutes early. I think 9:00 AM is the earliest they start because on my last session I arrived at 8:15 AM and the radiation tech told me the machines won't be turned on and tested until 8:45 AM.
It's always nice to have someone with you for a 5 1/2 hour drive home. Best of luck to you!
