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.

In Walk Two Moons, Sal joins her Gram and Gramps on a road trip to visit Lewiston, Idaho. She initially tells the audience that she is going to look after her grandparents, but then she goes on to list many other reasons that they are taking a journey across the country. The number one reason mentioned is "to see Momma, who was resting peacefully in Lewiston, Idaho." While the phrase "resting peacefully" is pretty specific, it is often overlooked. Readers often reach the end of the story and are surprised to discover that Momma is dead and we learned her fate in chapter two.
Sharon Creech gives us other clues about Sugar's death throughout the story in less obvious ways. In Mr. Birkway's class, the students read a poem about a man who disappears one night while walking by the ocean. Sal becomes obviously upset and assures her classmates that the man has died. This creates a discussion about death and how it is a normal part of living. Sal disagrees and states that death is never normal. Her strong feelings about the topic of death act as a clue to her mother's untimely end.
When Phoebe's mother disappears, Sal has a difficult time reassuring Phoebe that everything will be okay. She tells Phoebe that her mom may not come back at all. Sal knows from experience that sometimes things happen and people cannot come back, even if they want to return.
These are just a few of the many hints we have to help us uncover what happened to Sal's mother, who died in a bus accident in Idaho.