Question about the algorithm behind the 'suggest a ride' feature

Hello,

I use the 'Suggest a ride' feature quite often because I find that it works incredibly well! As a software developer, I can't help but be very curious about how the algorithm works behind the scenes.

I assume there is an internal graph representation of the roads/pathways, but I'm not sure what graph algorithms would be most useful from there. How are circuits found in the graph that are roughly around the target length and seem to be more or less circular so you don't end up going up and down the same road too much? It's so quick at calculating too!

Mark