In Robert Simons' original vision of Reston, walkers and cars would be separated; cars would have streets and roads, and walkers paths which went through the woods, instead of sidewalks paralleling the roads. That was the way Reston developed for the first 10-15 years, but then it became apparent that walkers preferred to walk by the side of the road, even when it meant walking on grass or in the mud, rather than following the path. So gradually Reston has added sidewalks to its paths (Colts Neck Road got a sidewalk south of South Lakes Drive just last summer.)
Why the preference? Often the roads are more direct than the paths. And the roads feel safer because you're visible to all. And we're all used to walking by the roads.
Our recent snow storm showed one virtue of Simons' vision: snowplows inevitably throw the snow from the street onto the sidewalk, creating an almost impassible barrier to cross, and a forbidding prospect to walk along. Meanwhile Reston Association is able to send a plow (small Cat, I suspect) down the paths and clear them off quite well, yielding to the weight of snow only when trying to break through the snowplowed-barrier.