Manholes are most commonly used to provide access to storm and sanitary sewer systems, which rely on gravity to move material through a gently sloping network of underground pipes that typically are the deepest of all underground utilities. Because these deep sewers serve both sides of the street, they operate most efficiently when placed near the center of a road, leaving room on the shoulder and sidewalk for shallow utilities like water, gas, electricity, cables, etc. Utilities need to be separated by certain distances for safety reasons and to reduce the number of times lines overlap. It would be too difficult and costly to reach a utility that’s aligned above or next to another utility, plus you run the risk of damaging one line while working on another. Also, despite the small appearance of the sewer manholes — which are 24 inches in diameter — the actual manhole foundation is much larger and wouldn’t fit beneath the sidewalk. Finally, while it does affect traffic when maintenance work is done in the middle of the street, it would also be difficult and inconvenient for pedestrians if this work were occurring on the sidewalk.
