Probably a good idea for the vehicle, especially if both involved in crash. However, EMT will do what they do in that circumstance anyway.Medical Power of Attorney. Ideally, filed at your hospital and doctor's office.
Here it is family/spouse decision when needed. I've been asked by hosp. if I had one (DNR), gave/give authority to heron the spot. When her mom passed and EMT called, they did ask if there was a DNR. Nope, but she said don't do it, verbal order by relative.
I had heart stop several times during surgery, they waited for a restart and it happened. Did wake up when they were ordering blood - guess it was for immediate open heart job? Lasted about a min, then back to nap.