Actually, the model used by the scientists is very flawed and therefor their conclusions are quite system-inherit.

If we consider the non-linearity of most bus tracks and how buses have to stop (they don't drive with a constant velocity!!), wait for traffic and traffic lights, then the equation and the solution to the question get quite complicated and there are more possibilities in which one would save time by walking a bit. It doesn't have to be on the track of the bus, but pedestrians can take shortcuts..

I've written my comment here:
http://tech.toomuchcookies.net/46/lazy-and-impractical-mathematicians/