Quote Originally Posted by m.eliza View Post
IMO the trail is slower because you need to be careful of the other people using it.
You need to be careful of other users on the road, too.

Average speed depends on many factors. Some of the slowest rides I've done have been on-road, because they were in urban areas with frequent intersections.

The trail in question here is 45 miles long, from Arlington, VA (just over the river from DC and heavily populated) out to Purcelleville, VA (less crowded suburbs/exurbs). Some parts are crowded with other cyclists, joggers, dog walkers, etc. while others are pretty empty.