Large triangle object moving slowly West from Duluth, MN

One evening in September of 1990 at about 10:30 PM I was driving towards Virginia, MN from Cloquet, MN. As I came up onto an overpass on Hwy 33 to the North of Cloquet and to the NW of Duluth, MN and saw about six vehicles parked on the side of the road with their hazard flashers on. Several people were out of their cars and were looking to the East. I stopped and got out to see what was going on. Looking to the East, I saw something very large flying just as it went overhead. It was silent and looked like a black triangle.
	As it passed overhead, I realized it must have been at least 200 feet wide and had some bright white lights on the tips that faced forward. It moved overhead and blocked out a considerable amount of the evening sky. It moved silently at about 30 MPH and was out of sight within a minute or so.
	Details about the evening: Approximately 10:30 PM (22:30), Sometime in September, 1990 (unsure of exact date as it was 16+ years ago), clear skies, new to ¼ moon (relatively dark), temperature in 50-60F range, no noticeable surface winds.
	Details about the location: Hwy 33, North of Cloquet, MN on Northbound bridge that goes over Saginaw Road (County Roads 694+46). This location is about 4 miles West of the Duluth International Airport and there is an airbase at that location, although not very active, just border patrols and Air National Guard.
	Details about object: Speed of approximately 30 MPH, altitude was less than 500 feet AGL, direction of travel was from East to West, visible for less than two minutes, triangle shaped with white lights on front (leading) edge, silent, no exhaust or emissions, no electrical interference.
	I am college educated geographic specialist and have researched some possibilities for this object. I didn’t really think about it for a number of years and finally thought about what it could have been. My first thought was that it was a stealth bomber, but realized that the actual production started later than when this was sighted. I’m sure there were prototypes around, but most likely not flying out of Duluth at this time. Stealth bombers and fighters have never been stationed out of Duluth. This object was also as silent as a bird; no wind noise, no jet exhaust, no turbine whine--nothing but silence.
	The fact that there were more witnesses confirmed that I wasn’t seeing things. I wish someone else would come forward and confirm this besides me.
Witness indicates that the date of the incident is approximate.  PD