Here's why:
* It names a thing: A road is a physical thing that we can see and touch.
* It can be the subject of a sentence: "The road was long and winding."
* It can be the object of a verb: "We traveled down the road."
However, "road" can also function as an adjective in some cases.
* Describing a noun: "They took the road less traveled." (Here, "road" modifies the noun "travel").
So, the part of speech depends on the specific context in which it's used.