Why are traffic circles circular instead of other shapes?

[harrycoolests]

why is traffic circle only circular and not of any other shape.i mean i have heard answers like it makes traffic flow smooth and stuff but i want to know the actual reason something which involves engineering sense.

 

[FredGarvin]

Probably because circles are easy to lay out and for a given area, a circle has the smallest perimeter.

 

[Bob S]

If the traffic circle has more than one lane, the circle is the only shape where the driver does not have to constantly adjust speed and steering angle to stay in his/her lane. By the way, around Cambridge, MA USA, what shape traffic "circle" is used in Harvard Square, Central Square, Kendall Square, Powderhouse Square, etc.?
[Edit] I attach a Google Earth view of Powderhouse Square (actually in Medford). It is neither a Square nor a Circle.

 

[brewnog]

If a traffic circle is a roundabout, it's probably because your American road layout men are lacking in imagination. They're all sorts of shapes over here. There's even one in Hemel Hempstead that you can go round clockwise and anticlockwise.

 

[mgb_phys]

Or this one in Swindon is almost fractal

 

[Ben Niehoff]

Dupont Circle in Washington, DC is pretty wild, too. There is an ordinary traffic circle, with a secondary circle inside it that is designed to more efficiently pass through traffic from a major road. I was riding through there two weeks ago, and it was quite confusing, having never seen it before: http://maps.google.com/maps?hl=en&ie=UTF-8&q=dupont circle&sa=N&tab=wl&um=1