At last count, there were over 2,300,534 of these in the us. while you can't see them, chances are pretty good you passed over 150 of them so far today. what are they?

Answer

City Blocks

Explanation

A city block is an urban block that forms a central element in urban planning and can be described as the smallest area surrounded by streets. They can be divided into small units such as private owned blocks. It is not possible to “see” city blocks because of their huge sizes.

81 is a composite number.