I saw the following representation in a paper that I was reading. Can anyone can shed any light on how it was developed? 

this is the [Paper](http://onlinepubs.trb.org/onlinepubs/shrp2/SHRP2prepubL02Handbook.pdf) - page 34

![Radial representation of Probability Density Functions][1]


  [1]: http://i.stack.imgur.com/CiCPw.jpg