Howdy- Tim Bevins again with the Park Service.
It is in fact a camera- one used for data collection. It allows for accurate figures on mode split between trail users as well as which directions people are headed.
At my request, VDOT was gracious enough to set up the counters for 24 hour counts at the 4 Mile Run and Crystal City connectors to the MVT. These are congested / potentially dangerous areas and this data will help the NPS develop interventions to alleviate the problems.
The one at Crystal City has already been taken down, and the cameras at 4 Mile Run should be removed from the trail on Friday.
Thanks!
Tim