Final answer:
The prepositional phrase in the sentence is 'across the closed highway.'
Step-by-step explanation:
The prepositional phrase in the sentence is 'across the closed highway.' Prepositional phrases begin with a preposition and function as adverbial or adjectival phrases. In this case, the prepositional phrase 'across the closed highway' provides information about the action of the verb 'rumbled' and describes where the huge dump trucks were moving.