I'm guessing each pallet is a 4x4x4... roughly. What's a couple of billion if I'm wrong anyway...
You would have a stack of $100's 4 feet high, 4 feet wide and about 7.5 miles long. Wikipedia lists the average walking speed between 2 and 3 MPH so if it's 2 MPH you could walk along the stack of hundreds that is 4 feet high and 4 feet wide for 2 hours and 45 minutes.
Wikipedia also lists the distance from the capital to the Lincoln memorial at 1.9 miles. You could have a stack about 14 or 15 feet wide is probably a little less than the width of a two lane road from white line to white line that would four foot high and wide. You could drive two cars down and and a motorcycle down it.