When dealing with a dangerous driver who refuses to give up the keys, consider telling your loved one that you'll report him or her to the DMV. Some drivers are persuaded to quit by the knowledge that the Department of Motor Vehicles may request in-office testing if it receives complaints. You can make this complaint yourself, if you think your loved one's driving skills are deteriorating fast; many states allow individuals to do so anonymously. The person is then called in for an on-road evaluation.
Ask your doctor to recommend a referral to the DMV or an occupational therapist who does driving evaluations. A local hospital, AAA office, or rehabilitation program might also offer this service.
One thing you can be sure of is that driving will continue to worsen: Driving is a complicated skill involving not just memory but many different kinds of higher-order thinking and physical responsiveness.
Study the driving laws in your state to see if any apply in your case to help get a driver off the road.