My mother doesn't have dementia, but is on MS Contin with Percocet chasers.  One day she took her meds twice without realizing it.  Then decided to drive one of the worst highways in NJ to go to a sewing store. She hadn't sewn in decades. Got off the wrong exit and stopped her car in the middle of the road in front of a police car. She was all out of it, he called EMS, she had low O2 sat but refused to go with EMS. My sister was called and went to the site. She asked the police to please report her to DMV, which they did. Took away her license, and for $100 she could retake the driver's road test. She failed 3 times and hasn't driven since. Sometimes you need outside agencies to help with this, so the family isn't perceived as the bad guy. We were very lucky.