Learn Russian: Большая машина едет вдоль нового шоссе. - A large car is driving along the new highway.

It is required by the preposition вдоль (along). Prepositions of spatial relationship like вдоль and вокруг (around) always take the genitive case. Because шоссе is neuter, the adjective новое changes to the neuter genitive form нового.

If the adjective is in the genitive case, why doesn't the noun шоссе change its ending?

Шоссе (highway) is a borrowed word, and like many foreign words ending in -е or -о (such as кафе, метро, and пальто), it is indeclinable. This means it never changes its ending, no matter what grammatical case it is in. We can only tell it is in the genitive case here because of the adjective нового attached to it.

How do you pronounce the ending of нового?

The -ого ending on adjectives is always pronounced as -ово (-ovo). This is a strict pronunciation rule for all masculine and neuter adjectives in the genitive case.

Why use the verb едет here instead of идёт or ездит?