If you're worried about your driving ability, contact a local driving school. Most will arrange for an instructor to take you out and go through procedures which may have been affected by slowed reflexes or limited vision, and point out any bad habits you may have developed. Or you may prefer to take the Advanced Driving Test; a pass can help to keep your insurance costs down.
Steer clear of potentially dangerous situations. Devise alternative routes to shopping centres and doctors' surgeries which avoid busy roads and junctions. If you are thinking of replacing your car, buy a white, beige or yellow one so that other drivers can see you more easily.
If you've stopped driving, look for alternatives: explore bus routes and scout out dependable taxi services. Many churches, senior citizen centres and clinics provide transport for older people. Contact your local branch of Age Concern for recommended pickup services.