Local norms that aren't always written down anywhere.
Horn use, following distance, and merging norms all vary more by region than most travelers expect, and matching local pace and habits tends to go more smoothly than driving exactly as you would at home. Observing how local drivers behave for the first few minutes of a drive is often the fastest way to calibrate.
In some regions, flashing headlights signals a warning; in others, it signals you're yielding — the opposite of what you might assume, so it's worth a quick check before you rely on it.