I was driving west on I-70 just outside Denver when I noticed an object left of my car, moving just below clouds. Object glowed bright white and while mostly round, the edges were not firm. It appeared to bulge in front sometimes. I must say that I was observing this in traffic on an unfamiliar road so I watched between glances at the road. It caught my eye because the glowing shape was clear in the twilight sky. As I watched, a medium sized airplane that must have been landing passed (in the same direction) much closer to me and below the object. This gave me a sense of the distance and the conclusion that the object must be quite large. The plane moved faster than the object and then crossed the highway in front of me. The object continued to move along, then was vanishing upwards into clouds when I looked again. I was sorry I could not get a picture without crashing.