There was a northbound helicopter being followed and circled by this smaller craft.

I was driving westbound on I-70 and saw a helicopter heading north. This helicopter was followed by a smaller craft which made rapid changes of direction and speed, circling the helicopter a few times but keeping mostly behind it. The smaller craft first looked oval in shape but became triangular, then back to an oval, and repeated this shift as they headed out of sight. The object was a darker gray, not quite black, easily seen against the partly cloudy sky.