No one can safely drive as dementia progresses from the mild to moderate stage -- not even someone who has moments of lucidity or who sticks to familiar routes driven for years. By this point in dementia, there's little room for equivocating or justifying.
Erosion in judgment and other thinking skills make safe driving impossible as dementia progresses. Even if the person seems to know by autopilot what to do behind the wheel, this is deceptive. A driver with dementia is a hazard to other drivers and pedestrians on a par with a drunk driver.
Don't wait until it's too late -- an accident has occurred -- to take action. It's time to consider these drastic measures:
Contact the Department of Motor Vehicles. Cite your concerns, including specific examples of poor driving, and give your contact information. In some states, you can register a complaint about an unsafe driver anonymously. An evaluation of driving and possible revocation of the license will follow. Find out the state laws where you live.
Get the person's doctor involved. In many states, a doctor treating someone for Alzheimer's disease has a legal obligation to order the patient to stop driving and/or to report the person to the DMV.
Take the keys and/or disable the car. Ask a mechanic how to do this. Or have the tires "stolen" (removed).
Sell the car if you have legal authority to do so. At this point in dementia, someone should have power of attorney to manage or comanage financial matters such as this.