Don D. sends along a item noting the sniper rest area was within view of the foot bridge where the Appalachian Trail crosses I-70 near Exit 42. It was on the Westbound (north) side.
Here's a topo map:
The AT is on the left by Washington Monument State Park:
http://topozone.com/map.asp?lat=39.505&lon=-77.56667
Myersville Appalachian Trail on web:
http://www.google.com/search?sourceid=navclient&ie=UTF-8&oe=UTF-8&q=myersville+appalachian+trail