In downtown Troy in recent times, at the intersection of River, Third and Fulton, they got fancy and added cobblestone for the pedestrian crosswalk. What a great way to strain your ankle.
Cobblestones are rocks found in streams, rounded by the action of water.
What we generally mean by a cobblestone street is one that was actually paved with what was called Belgium paving blocks, stone cut into rectangles, which gave a smooth surface. Sufficient traffic wears the corners of the blocks, rounding them, as can be seen on a street next to Washington Park in downtown - the stones in the center of the street are rounded, those near the curb are not. The blood flow of a urban area is not the street traffic, but the pedestrian traffic. These are the potential shoppers, who will walk into various stores. As feet are narrower than the rest of the person, the slabs are spaced apart by a couple of rows of paving blocks.