Find the simple subject and simple predicate:

Traffic moves slowly over our modern superhighways.

Traffic = simple subject

The simple subject is who or what the sentence is about without any modifiers that describe it. In this sentence, the sentence is about traffic.

moves = simple predicate

The simple predicate is the main verb. Main verbs can be action or linking. In this sentence, the main verb is moves and it is an action verb. It tells what the traffic is doing.