A heavy metal lid in the road seems an unlikely object for a famous design puzzle. Yet the question has a satisfying answer: a round cover offers practical advantages because its width does not change when you rotate it.
The word “usually” needs care. Streets also contain square, rectangular and other access covers. Different openings and maintenance tasks call for different designs.
A circle has the same width in every direction
For a circular cover, every diameter is equal. If the cover is correctly sized to overlap a smaller circular opening and rest on its frame, rotating it does not reveal a narrower direction that can slip through.
A square is different. Its diagonal is longer than its side, so a square opening can offer a larger span in one direction. Depending on the dimensions and supporting frame, a square cover tilted and aligned diagonally may pass through where it would not lie flat.
The important qualification is the complete geometry. A cover needs an appropriate bearing surface and overlap. “Round” is not a substitute for correct sizing, structural strength or a secure installation.
Rotation does not create an alignment problem
A circular lid can be seated without matching corners to corners. That makes orientation less fussy during maintenance. Some designs add hinges, locks or other features, so their handling still follows a specific procedure.
A round object can also be rolled rather than carried for every part of a move, although real access covers are heavy and require proper handling equipment and practices. The geometric possibility is not an invitation to move one casually.
Imagine fitting a round dinner plate into a matching circular recess. You can turn it freely and it still fits. A rectangular tray has to meet its recess at the correct orientation. The everyday comparison captures one of the design conveniences.
Why not make every cover round?
An opening may need to admit equipment with a rectangular footprint. A service chamber may require a large access area, or paving and existing structures may favour a different form.
Manufacturers therefore offer many cover and frame arrangements, including divided and hinged designs. Engineers choose load capacity, material, security and access dimensions alongside shape.
There are even non-circular mathematical curves of constant width. The circle is not the only shape with that property, but its simplicity makes it an obvious practical option.
A good answer is bigger than the puzzle
The familiar explanation—“it cannot fall through”—captures a useful geometric advantage for a properly designed round cover and opening. It does not explain every access cover in every city.
The fuller answer is that circular geometry combines predictable overlap with convenient orientation and handling. What appears to be an arbitrary street detail is a small example of design matching a shape to a maintenance problem, while leaving room for other shapes when the problem changes.
Sources
Wolfram MathWorld: Curves of constant width
Saint-Gobain PAM: Access covers and gratings product brochure