Cars symbolize the private order that Christopher puts on the world to make it intelligible to him. Under Christopher’s rules of his own creation, the colors of the cars that he sees on his bus ride to school indicate whether he’ll have a good day or a bad one, just the way the weather does for many people. Christopher in fact uses the cars as a sort of talisman of good or bad luck—if he sees the wrong color, he won’t speak to anyone all day, and if he sees the right color, he’ll take more risks because he believes they’ll turn out well. He even draws a number of red cars on his mother’s get-well card when he thinks she’s in the hospital, hoping that the cars will help her get better. In this way, they become almost like a religion to Christopher, who doesn’t believe in God.
Once Christopher reaches his mother’s house, he tries to determine what sort of day it will be by watching the cars out the window, but he realizes that this doesn’t work because he can watch for as long as he wants and see a number of colors with conflicting meanings. At this point, he has to give up on this set of rules. As his world expands and he grows up, Christopher is forced to lose certain illusions, like his prophesying cars. In doing so, he becomes more fully a part of the world around him.