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?

City blocks.

Step-by-step explanation:

A city block, which is also known as an urban block or simply called as a block is the smallest area that is surrounded by streets. City blocks are usually the spaces for your buildings within the streets of a city, and form the basic design of a city's urban area. City blocks can be further divided into any number of smaller areas which are usually in private ownership, but in few of the cases, it can be other forms of tenure.

City blocks, if you only include blocks that are in what is designated as developed land by the census. You can't "see" them due to their size. I can't find a census on how many city blocks there are in only developed land in the U. S. (the census also counted rural blocks which puts the total at above 11 million), but doing some math and using estimates lands us in the ballpark of 2,300,534. There are 3.797 million square miles in the U. S. A., of which, about 3% is considered developed land. 3.797 million * 3% is 113,910. City block sizes vary, but on average there's somewhere between 16 and 20 city blocks per square mile. We'll use 20 and assume that the exact total provided to us isn't super accurate. 113,910 * 20 is 2,278,200. Counting the blocks you pass on both sides of the road and assuming that you'd pass 8 city blocks per side of the road per mile, you'd only need to drive 10 miles to pass 150 blocks.