Watch Your Head
Jean Louise must have a goose egg on her noggin—she remarks on bumping her head, or almost doing so, three times during the novel. Come on, JL. Do you need a crash helmet?
The first is when she gets into Henry's car, and "she bumped her head hard against its top" (5.1). This prompts her to complain that car roofs are getting lower and "We'll be riding prone next year" (5.5). Jean Louise sounds like your uncool grandma complaining about "learning the Facebook." (Meanwhile, cool grandma has more Instagram followers than you do.)
Later, when leaving the grocery store:
She bumped her head getting into the car. I shall never become accustomed to these things. (12.131)
This is the Jean Louise equivalent of uncool grandma forgetting to charge her phone after being told she has to it every day.
Finally, in the book's final sentence, she wises up to the height of the dang car roof:
[…] she went around the car, and as she slipped under the steering wheel, this time she was careful not to bump her head. (19.29)
What does this all mean? Criticism of Henry Ford and the Detroit auto industry? Is Jean Louise extremely tall? Is her head the size of a watermelon?
Since Jean Louise's main conflict is dealing with change, particularly change in her father, the car roof seems to be equivalent to Atticus. Jean Louise unexpectedly hits her head on Atticus (metaphorically) a couple of times before she learns to accommodate herself to him. But while she complains about cars changing, by the end of the book, she's become accustomed to Atticus… who never changed.
Deciphering this muddled car-roof metaphor is harder than learning how to drive.
The most important thing to note here is that Jean Louise has to accommodate herself to both the car and Atticus. Atticus is as rigid as the metal roof of the car. He will never change. So, Jean Louise bends her neck and submits to unbending roof of the car…while at the same time submitting to the unbending racism of ol' A.F.