He starts saying, "Stop! Stop! Stop!" long before I get to a signal or stop sign. He breathes through his mouth like a slow hiss the entire ride. How can I get him used to not driving?
Perhaps it's best for now that you leave him at home with a sitter/aide while you run your errands and only take him out if absolutely necessary, like a doctors appt. And then you can always turn up some nice soothing music when he's in the car with you, in hopes that it drowns out his noises, and keeps you both calm. Good luck!
To be really frank fear isn't limited to those who are elderly OR suffering from any dementia. When you are driving you are in control.When SOMEONE ELSE is driving you are not in control. I find myself putting on the brake on my passenger side of the car. So this is a fairly normal reaction, exacerbated by his dementia. Hope he becomes used to it. The only answer may be to have him do some puzzle or drawing other than watching the road. Otherwise he may need to be, indeed, a "backseat driver". He will be less bothered if he rides in back of the car.
And then you can always turn up some nice soothing music when he's in the car with you, in hopes that it drowns out his noises, and keeps you both calm.
Good luck!