I spotted an unusual aircraft on Thursday, September 6, 2012, approximately 9:45 PM in Austin, Texas. I was traveling West on the 2222 and had just crossed over the 360 when I spotted the aircraft traveling E/NE. The aircraft was at first was in the distance, flying very close to the hills, and I noticed it was an abnormal size and had strange lights for an airplane, it was also moving extremely slow. As I continued on the 2222 the aircraft flew closer to the 2222 and I realized it was flying extremely low and the slow speed became more apparent. It passed over my car and I observed it was a solid triangular shape with brilliant green and red lights along the outside of the triangle. I cannot recall the exact pattern of the lights. What struck me the most odd, and why I am reporting this, is the triangle shape. It was a solid triangle, with no visible wings.I lost sight of the object because it passed over me, continuing NE and I was heading home, directly West.I am not a UFO believing person, but I this was so unusual and thought to myself, "I think I just saw a UFO!" I did some research on military planes and could find nothing to matched what I saw, unless of course it was a secret plane.