How do Logan children watch the trail and why?

The Logan children watch the trail on their way to school because the school bus for the white children speeds dangerously down this road.
In Chapter 1 of Mildred Taylor's Roll of Thunder, Hear My Cry, a novel set in the Jim Crow South, it is the first day of classes and the three Logan children are on their way to their school. Cassie has promised her mother, who teaches at the school, that she will arrive "looking clean and ladylike." It is unnecessary for Mama to remind Little Man because he is obsessively neat on his own. In addition to obeying their mother's order to arrive at the Great Faith Elementary and Secondary School in a presentable manner, the Logan children watch the narrow winding red dirt road with its deep gullies because they know that the school bus driver races down this road toward the Jefferson Davis County School.
When they hear this bus, the Logan children try to get out of its way. Little Man, the youngest of the Logans watches "saucer-eyed" as the bus bears down on him, sending clouds of the red dust from the clay road into the air. He attempts to climb the bank of the road, but he cannot find a foothold in this steep bank. Laughing faces inside the bus press against the windows of the bus, watching the dirt cover the Logan children's clothing.
Little Man is too young to understand the gratuitous cruelty of the bus driver and the white children. "How's come they did that, Stacey, huh?" he innocently asks his brother. This incident, along with their friend T.J.'s mention of the terrible burning of the Berry men by night riders, sets a tone for the narrative.