If it truly is dangerous for your mother to drive now, you need to act. If you don't, her life could be in danger, and so could others.
However it won't be easy; decisions at that age seldom are. Remember: Her car means far more to your mother than just the ability to get around. It gives her a sense of freedom and independence, and taking away her ability to drive means taking away that feeling. It has practical implications, as well - the need to rely on others to take her to the grocery store or the doctor, for example.
Probably the only way to handle this difficult decision is with both compassion and firmness. You need compassion, because this will be a major turning point in your mother's life, and it will be hard for her to accept. How would you feel if you were in her position (as you may be someday)? However, you also need firmness, explaining to her why the time has come for her to stop driving, and urging her to make the decision on her own. The Bible says, "Clothe yourselves with compassion, kindness, humility, gentleness and patience" (Colossians 3:12).
Go out of your way also to assure your mother of your love - and of God's love for her, as well. God's promise is true: "Even to your old age and gray hairs I am he, I am he who will sustain you" (Isaiah 46:4).