I was followed closely by a flashing blue light, for about 15 miles.I was driving between Jayton and Post on Highway 380 on a very hazy, humid night. I had never travelled this highway before, but was told later that it was remote, unpopulated ranch land. As I approached a rise, I could see the reflection of a flashing blue light in the haze on the other side of the rise, but could not see the light itself. I assumed it was the flashing light of a communication tower. When I topped the rise, to my left was a flashing blue light about tree top height over the fence line, right next to the highway. To my astonishment, the light, which appeared to have been hovering, immediately went from stationary to 65 miles per hour, traveling still over the fence line next to the highway, as though looking at me through the drivers side window of my van. The light was bright blue, about the size of a basketball, flashed about every second, and I heard no sound from it. For several minutes the light stayed right beside my van. I thought maybe I was beside a train or something, but later found out there were no tracks beside the highway. The light drifted behind me, and I slowed down to do a u-turn to try and figure out what I had just seen.As I slowed to about 10 miles an hour and was about to turn around, the light appeared outside my drivers side window, again as though staring at me through the window. Now I am scared, and I jammed on the gas, only to see the light accelarate at the same speed I was going. For the next 15 or so miles the light travelled right beside, behind, or over my van. At times I could see it over the highway in my rear view mirror. At other times I could not see the light itself, but could see the flash reflected in the haze, realizing that it was right over my van. It would also move over the ditch on the other side of the highway(Blowing my theory that I was beside a train running on tracks beside the highway). As I approached the city of Post, the light finally disappeared from view. I reported the incident to a deputy sherriff I saw coming out of a cafe, assurring him I was not some crazy person or high on something. He said I looked and sounded credible to him, and said he would drive out later to take a look. He told me that there were no train tracks beside the highway, and that I had driven through ranch land that was largely unpopulated. He had no clue about what I had just seen. I am a conservative, professional person, late forties in age, and don't do drugs or drink. If this had not happened to me personally, in the past I would never have believed this story to be real if I had read it.((Addendum by same source: A flashing blue light flew beside and behing my vehicle at tree top level for 15 miles.))((NUFORC Note: We have attempted to contact this witness at the e-address he indicates, but the address is not valid. We would very much like to correspond with the witness. PD))