Learn Hardcore Russian: Осторожная водительница медленно едет по узкому шоссе. - A careful female driver is driving slowly along the narrow highway.

Gender: водительница is <a class="border-b border-dotted border-blue-400 dark:border-blue-500 cursor-pointer text-inherit no-underline" href="/grammar/russian/nouns/gender/overview">feminine .
Number: singular.
Case: <a class="border-b border-dotted border-blue-400 dark:border-blue-500 cursor-pointer text-inherit no-underline" href="/grammar/russian/cases/nominative/forms-and-subject">nominative (it is the subject).

The basic form of the adjective is осторожный (careful). For feminine, singular, nominative it changes to осторожная (ending -ая) to match водительница.

Why is водительница used instead of водитель?

Водитель is grammatically masculine and usually means driver (gender-neutral in many contexts, especially if gender is unknown or not important).

Водительница is the explicitly feminine form and clearly means female driver:

водитель
- feminine suffix -ниц(а) → водительница

So the sentence wants to make it clear that the driver is a woman.

What case is по узкому шоссе, and why is that case used?