(The reason why manhole covers are round is obvious - because manholes are round (which also happens to have additional benefit of not allowing cover to fall into manhole). So the question is why manholes are round?)
Manholes are round because of the same reason most wells with brick walls are round - less material is required to support pressure trying to collapse the shaft. I.e. given that manhole needs to reach the same depth, it will be cheaper to have round shaft than, lets say, square, because round shaft walls can be thinner, but still support required pressure.
Surely if it makes sense to have manhole shafts be round, and it also makes sense to have manhole covers be, say, square (for whatever reason), then manhole covers would be square, no?
1) Covers also can have hinges, so training of personal may be not even required.
2) It's unclear that round cross-section is the most convenient for humans to climb, after all humans in horizontal cross-section are like elongated ovals, so rectangular shape could be better. :)
A cover in the shape of a reuleaux triangle still wouldn't fall into the shaft, but also wouldn't roll away--it would have constant width, but its center of mass would move up and down while rolling. Apparently the UK has superior civil engineers.
In modern US cities there are a lot of different manholes, mostly round and rectangular - some shafts don't go deep, so there is no need to make shaft round.
I wonder what kind of shaft is under triangular cover, why would someone build triangular shaft? :)
Manholes are round because of the same reason most wells with brick walls are round - less material is required to support pressure trying to collapse the shaft. I.e. given that manhole needs to reach the same depth, it will be cheaper to have round shaft than, lets say, square, because round shaft walls can be thinner, but still support required pressure.