Saucer/disk shaped aircraft spotted just north of Lawtey, Florida

We were driving down U.S. Route 301 headed from Tampa to Jacksonville when a large saucer crossed over the road in front of us headed from east to west. It was large enough that it took up the width of the entire four-lane road and median, and the ends of the saucer were both over the trees. It was 100 ft in the air, just above the tree line. It was a grey, round disk - almost like an upside-down frisbee. It was completely smooth on the bottom and dimly lit around the edges - not individual lights, but lit all the way around. It wasn't going very fast at all. It moved slowly and smoothly across the roadway. A few seconds after the disk crossed a helicopter followed it across the road. The helicopter was significantly smaller than the disk - a very small fraction of the size.  Observers are two nurses - one male, and one female - both in our 50s. We turned down a sideroad to attempt to follow it, but we could not keep up.  Our coordinates when we stopped drivin! g were 30.0813, -82.0747 on NW 251st Street.