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Manholes are round to accommodate their covers, which are round for several reasons. First of all, it is so they don't fall in the open man hole. Secondly, being round, they can be easily rolled, making them easier to move than if they were square or some other shape.
Here is a website with more information on manholes and their covers: http://www.improvedconstructionmethods.com/why_are_manholes_round.htm
I hope this answers your question, but if you need more information please let me know.
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The safety factor enters the picture, because the round covers cannot drop down into the manhole. They will not fit, no matter how they are positioned. Therefore, the workers below are protected from the cover being inadvertently knocked into the manhole and injuring them.
Another shape of lid (such as oval, square or rectangular) can drop into the manhole and potentially cause injuries to those working in the manhole.
I hope this better answers your question.