Here's why:
* Common nouns refer to general categories of people, places, or things.
* Proper nouns are specific names of people, places, or things and are always capitalized.
Examples:
* Common: street, city, dog, book
* Proper: Main Street, New York City, Spot (the dog), Harry Potter (the book)
Therefore, "street" is a common noun because it refers to a general type of road, not a specific one.