I should have posted this one a very long time ago. Reviewing another post reminded me that not everyone traveling through will know this. It you drive through the South, especially in Alabama, and you’re not from the region then it may seem a bit strange your first time.
The answer isn’t a major mystery.
They’re just being friendly.
Wave back, if you have a chance. It doesn’t require a big wave. Raising a couple of fingers off the wheel for a second or a nod is just fine.
And don’t worry if you miss it or just don’t feel like waving back. Nobody’s going to turn around and corner you (that’s a form of thought caused by very bad movie stereotypes).
It’s just a common courtesy.
And yes, the S is capitalized on purpose.