Triange hovers over highway, moves at increasing speed over highway then disappears

On a late sat. evening, near national guard firing range east of Helena Montana (on way to Townsend), I drove under triangular shaped object w/ white lights on each corner.  I Assumed at first that it was a helicopter over the highway as many fly around the range, we also see alot of parachute flares when the guard trains at night.  When realized the object was a few hundred feet over the highway, I stopped van, got out w/ friend and looked up at the object that emitted only what sounded VERY faintly like an aircraft APU.  The object remained directly overhead for not more than 20 secs and then tilted forward, moved slowly at first over the highway in the direction of Helena.  My friend and I followed the object in the van speeding at one point up to 100mph for not more than a minute then the object seemed to disappear (lost in the city lights of East Helena).  We drove to the Helena airport and saw no activity there.  The next day we returned to the Helena airport (in the daylight) and saw no strange aircraft but a few USAF cars - as far as I know there is no AF presence in Helena.  My friend an I were basically laughed out of high school when we told a few friends and our parents...although one friend had an uncle who was a veterinarian that seemed interested because of some strange cattle mutilations that had happened in the area... I put this out of my mind as something that I could never talk about but a coworker mentioned about flying triangles after seeing some t.v. shows regarding this subject.  After looking on this website, I noted that another gentleman in the Helena area had a similar experience in broad daylight..I would be very interested in talking to that individual.
Witness indicates that the date of the incident is approximate.  PD