I've been sort of monitoring this phenomenon for a while now and I notice a 2-3 MPH drop when I'm on a path as compared to when I'm on the road. This seems odd because many of the paths around here are fairly flat and the roads have some hills. Disregarding the 15MPH speed limit and even on very sparsely populated days my average speed on a bikepath is somewhere around 18MPH whereas on the road it's somewhere between 20 and 22 or even higher if the roads are flat. And yes, there's the issue of peds and slow cyclists on paths but even on those days where I encounter absolutely no one, my average seems lower. I wonder if it has to do with the surface grade differences. Maybe paths use a softer surface thus there's more rolling resistance.