Why are manhole covers round?

Ok, so I was reading an article on strange interview questions. So I decided to post one on my facebook(I spend too much time there) and my friend Michelle told me that she was once asked,"why are manhole covers round?". Very interesting? My first thought was because the man hole is round...but I thought there had to be a more explainable answer. So I looked it up.

Turns out. I'm pragmatically right. That means I took the simplist answer. but there is more to that explanation as well.

A round manhole cover cannot fall through its circular opening, whereas a square manhole cover may fall in if it were inserted diagonally in the hole. (A Reuleaux triangle or other curve of constant width would also serve this purpose, but round covers are much easier to manufacture. The existence of a "lip" holding up the lid means that the underlying hole is smaller than the cover, so that other shapes might suffice.)

Round tubes are the strongest and most material-efficient shape against the compression of the earth around them, and so it is natural that the cover of a round tube assume a circular shape.

Similarly, it is easier to dig a circular hole and thus the cover is also circular.

The bearing surfaces of manhole frames and covers are machined to assure flatness and prevent them from becoming dislodged by traffic. Round castings are much easier to machine using a lathe.

Circular covers do not need to be rotated to align them when covering a circular manhole.

Human beings have a roughly circular cross-section.

A round manhole cover can be more easily moved by being rolled.

If a cover had corners and were bent that would create a protruding point that could puncture tires.

Most manhole covers are made by a few large companies. A different shape would have to be custom made.

I'm personally going to also add that there has to be an aerodynamic advantage to having a round manhole cover in a street, where a flat side would be more likely to catch wind from passing cars and such and potentially cause it to fall through the hole. Not likely it would lift off as the weight is substantial.

So, I knew there had to be a reasonable set of explanations. However, there are manhole covers in many shapes and sizes.
http://en.wikipedia.org/wiki/File:Heroid_Shehu_FreeSB_2010_100_5701.JPG
http://en.wikipedia.org/wiki/File:Tombino_108_Milano.jpg
http://en.wikipedia.org/wiki/File:ManholeSPQR.JPG
http://en.wikipedia.org/wiki/File:Vindobona_Hoher_Markt-71.JPG

I just blew my theory on the car and aerodynamics;

Manhole covers and race cars
Because of their aerodynamic design, modern racing cars create enough vacuum to lift a manhole cover off the ground. During races on city streets, manhole covers must therefore be welded down to prevent injury. In 1990, during the Group C World Sportscar Championship race in Montreal, racer Jésus Pareja's car struck a manhole cover that was lifted by the ground effect of the car he was following, causing his car to catch fire.