I saw the wood-slat fencing around Gravelly Point (by National Airport) the other day. It separates the grass field and picnic areas from the pavement of the Mt. Vernon Trail. It's a bit of an eyesore but I think it increases safety in that area. A lot of kids and adults tend to wander onto the trail and block off access to cyclists, runners and walkers at that spot. This is especially hazardous at night. Most people don't wear reflective gear or use headlights/taillights so it's very difficult to see people on the trail.
Does the National Park Service install this fencing every year? I don't remember seeing it last year.