I was returning home after spending time in West Phoenix and found myself turning onto southbound I-17 at the Thunderbird Ave. onramp. This was at approximately 9:28pm Arizona Time on Saturday April 23, 2016.Quickly after entering the freeway I had noticed what looked like helicopter lights and the helicopter was hovering. It looked rather high in the sky and it was of course a dark night so I was unable to classify the craft, other than observing the lights at this point, and its unusually high position in the sky over the N, NW sky of Phoenix.Next I observed (of course this is all while I'm driving on the freeway) a very dim green light descend from this craft. As in falling from the hovering craft. It appeared to fall more than 100 ft. Then the green light stopped in mid fall. And appeared to hover below the original craft. I could tell the craft above was moving westward because the two no longer seemed to line up in a straight line. After about 20 seconds, that's when things became interesting. The green light that had been hovering after falling instantly turned into a much larger object that I only could describe as a bloated double stuff oreo cookie. It was impossible to see the top and bottom of this craft as it was black against the night sky. The remarkable (and visual part of this siting) was the pale white center of the craft. It did not emit light but rather looked like light was reflecting off of it. Pale white, not too terribly bright. It appeared right from the green light that had fallen and hovered from the original craft (bright lights, strobing, moving).Keeping in mind, I was able to see this "bloated oreo double stuff shaped" craft for what seemed like a minute or two while driving. I wished I had pulled over, but again I was on the freeway. I was hoping there would be many other sightings and we could figure out what this was. It indeed was a UFO, if it was a hoax it was superb!If you have any information which might clarify this siting, please do not hesitate to contact me at my information provided.