Final answer:
The participle phrase in the sentence is "driven by a teenager," which acts as an adjective modifying the noun "car."
Step-by-step explanation:
The participle phrase in the sentence "The car, driven by a teenager, was speeding down the street" is "driven by a teenager." This phrase functions as an adjective modifying the noun "car" by providing information about who was driving it. As with other participle phrases, you can move it to a different part of the sentence for stylistic purposes without changing the basic meaning. In the context of this sentence, the phrase is necessary to keep as it provides crucial information about the subject, the car.