What clues in Walk Two Moons reveal what happened to Sal's mom?
There is another clue that reveals what happened to Sal's mother.
The clue is evident from a flashback conversation between Sal, her mother, and her father.
In the conversation, Sal's mother said that she was planning to go all the way to Lewiston, Idaho. Upon hearing that, Sal's father asked how she was going to get there. In response, Sal's mother said that she was going to take a bus. After hearing her response, Sal's father told her that she wouldn't have control over where she was going and how fast the bus would go. This is a huge clue as to how Sal's mother died.
The warning Sal's father gave was significant because Sal's mother was terrified of cars and didn't always like the speed at which vehicles moved. In other words, Sal's mother felt that people often drove too fast. By taking a bus (driven by someone else), she would have had no control over the speed of travel.
So, taking a bus all the way to Idaho would have been a risky action for Sal's mother to take.
Another clue as to how Sal's mother died can be seen in what Sal reveals about the road to Lewiston, Idaho. She tells us that the road to Lewiston curved in treacherous ways:
Maybe I could hitch a ride with someone who drove extensively fast, but the thought of someone speeding, careening around curves, especially the snaking curves down into Lewiston, Idaho, which I had heard so much about—when I thought about that, it made me dizzy and sick.
The clues here support what is eventually revealed in the novel: Sal's mother died in a bus accident.