What does Sal worry about as as Gramps drives along the winding mountain roads in Walk Two Moons? Why is she so nervous?

Sal worries about her grandfather's driving both because he drives quickly and because her mother recently died in a vehicular accident.
Sal says that her grandfather drove through Wyoming like "a house on fire." They go through a variety of terrain, but he doesn't slow down. He's focused on getting to where they need to be. Once they see the geyser, they go to Montana. Sal says that she didn't realize the route through that state would be all mountains—and it makes her scared enough that half of her attention is focused on the twists and turns of the road.
Sal's mother died when her bus went off the road, flipped, slid through an overlook, and slammed into a group of trees. A man who tells her about the accident says that when night fell, the rescuers were still cutting through the brush to reach the injured. Since her mother died, Sal has been scared of car accidents and death. These fears are exacerbated by the drive through the mountains with her grandparents.