Silent, slow moving black triangle hovering over interstate.

I lived with my parents in rural Pennsylvania about 2 miles from town.  A bridge on Interstate 80 passes over the road we lived on approximately 1/2 mile from our family home.   I was returning from town just after dark with two of my brothers in one of their trucks.
As we stopped in front of our house we noticed lights hovering over the interstate.   We decided to get  a closer look at the lights.  We drove about half the distance from our house to the interstate (1/4 mile).  The lights started drifting through the sky slowly toward us.  We then turned around and drove back toward our house, losing sight of object.  When we arrived at the house our dog was barking furiously, which was uncharacteristic for him.
We exited the truck and looked up.  Approximately 300 to 400 feet above us was a black triangle approximately 40 to 50 feet on each side.  There was a light at each corner (I think red).  It made no noise.  We just stared up at it for approximately 30 seconds.   It then slowly moved away making no noise until eventually it went out of sight.