My mother has become a very timid driver as she's aged. I don't want to limit her mobility but I worry about her. What to do?

She may simply be smart. She may rightly realize that her skills are limited now and fear is her motivator. If that's the case, ask her where she's comfortable driving and where she is not. Then try to take her places where she is afraid to drive and/or help transition her to senior ride vans (through senior centers), and other types of transportation. This is actually less of an issue than someone who insists that they are "fine" when they are not. Encourage her to share her concerns. Maybe a doctor can help with whatever issues she is having and she'll be fine for awhile, but if not, she needs to transition to safe a reliable transportation.
Carol