Driving

The old joke goes, "When i die, I want to go peacefully in my sleep like Grandpa did. Not screaming in terror like the other people in his car!"
Well as anyone with an aging relative knows, driving is a huge point of contention. Aging reflexes slow, attention span shortens, fear of death wanes whatever the reason that causes the elderly to change their driving styles it can be truly terrifying for those along for the ride. As I write this I am in fact in mortal peril. Grandma is driving and I have Metallica playing on the radio just quieter than her hearing will discern any of the lyrics. We are on the highway and it seems that all lanes are now fair game. She brakes when she feels she has started to go a little too fast downhill. Pity to the minivan cruising behind us when grandma suddenly decelerates from 87 to 70 in the space of a few seconds. My seatbelt is welcome company. Even more confusing than the abrupt braking is the acceleration that follows. Brake, coast, speed up only to brake again, repeat. We are now pushing 90 and I am sorry to anyone in my life I have not made amends to.
The problem is, when do you put your foot down and say you may not drive with me in the car anymore vs. you may not drive period. While my concern extends to the other people on the road, it is mainly centered around my own well being.
I will stop a drunk friend from driving but I cannot stop every drunk. Should I stop my grandmother from driving for much the same reason? Part of me says respect your elders and respect her decisions as those of any adult to direct their own life, part of me says call the state police and get me out of here!