It hovered until we stopped like it was waiting for someone to see it.We were headed east out of rapid city at exit 59 onto I90. I was driving and noticed a set of landing lights pointed west like they were landing at Ellworth airforce base. After a mile or two I realised they were'nt moving and said so to my companions. We continued east all the while watching the lights. When we got to the last airbase exit I stopped the car at the side of the off ramp. We all three got out and looked up directly above us at this large black triangle. It was just hovering above us at about 200 feet. My guess was about 60 feet from corner to corner. It continued hovering there for about 20 sec. and then 1 corner went down, another went up, and it turned and floated south towards Rapid City Regional Airport where it disappeared.The weird things: It was right along side the flight path of military planes landing which is s.e. to n.w.. It was just after sunset the sky was somewhat dark but it was still plainly visible against the light sky. We were the only people who stopped and I got the feeling they were waiting to see if anyone would stop and look. When it turned we were directly below and I heard barely audible very muffled jet engines. When it flew towards the civilian airport it disapppeared. At first we thought it had to have flown over the airport but after thinking about it we knew that wasn't likely as there was a plane landing at that time. The civilian airport is up on a hill which gave me reference height and we believe it flew over towards there and disappeared in front of the hill. How we don't know. It had landing lights on it just like the military and civilian planes all do. No other lights were displayed. Distance from exit 59 to the last exit is approximately 7 miles. It was facing straight west while the landing path is to the n.w.. I think it was a military plane as the airforce base radar surely would pick it up.((NUFORC Note: Witness indicates that the date of the incident is approximate. PD))