The car is moving too quickly. Here, the word too is a/an:

- Here in this sentence, too is functioning as an adverb.
- the adverb too is modifying the adverb quickly, which are working together to modify the verb moving.

An Adverb can Modify:
- a verb
- an adjective
- another adverb
Adverb answers the question:
- How, where, when, why, in what manner.
Ex: Today he was unusually happy. [ Both today and unusually is an adverb here]

The role of an adverb may be played by group of words: a phrase or a clause. This is called an adverbial phrase or adverbial clause. i.e.: All quiet on the western front.

Source: Intermediate English Grammar by Raymond Murphy