Saw this paper at SBIM 2008. Sketching Piecewise Clothoid Curves http://www.dgp.toronto.edu/~mccrae/clothoid/

It may be very similar to how Spiro is implemented, but last time I looked at Spiro I couldn't find any sort of an algorithmic description. Anyway, the approach presented by this paper seems pretty simple and logical to me, so maybe it could suggest some ideas for Spiro. Or not.

It's a good read either way. The video is nice too.

--bb