Is she usually OK with driving in any other situation? My grandparents are now reluctant to drive to my aunt's place (the closest relative they have, almost in the same city) because neither of them feel comfortable driving at night, or in the rain, or in the snow.. anymore - they just never said it like that at first so we didn't understand why they wouldn't visit anymore. Maybe something to consider.
My ILs are still independent and drive themselves all over the place, so that's not the reason. She is just controlling and can't understand why people won't do what she wants them to do.
Another brain-hurty courtesy of my MIL:
We were at our house because FIL was helping DH with a project. MIL came along because she just can't believe that they would do it right without her there (her own words, not mine). The project took longer than planned and it was close to dinner time, so I said I was going to start preparing dinner. MIL is very picky about her food, but we had bought some stuff from an Asian supermarket that we know she likes. MIL got this pouty face when she heard the plan, and suggested that we bring the food and drive back to their house instead.
Let me get this straight - you wanted us to take the food that we already have in our house, drive all the way back to your house (45 minutes away), just to eat dinner there? When we are already here at our house with the food? Yeah, wasn't going to happen. She was unhappy the rest of the night simply because none of us was willing to give in to her crazy plan.