Pretty sure, there's a Helicopter Chart for the DC area. Just go to the FAA webdite and download it.
DC isn't like everywhere else in the US. Its very restricted airspace requiring "essentially" an IFR like setup to fly through. Everyone gets a unique squawk and must be on an approved flight plan and frequency.
The accident in question was a military aircraft on NVG that had some kind of altitude discrepancy (maybe they had the altimeter set wrong?), plus at low altitude at night, its easy to mistaken one light for another, thinking you have someone in sight when its really someone/something else.