My husband and I were driving south on Interstate 29 going to Sioux Falls. When we were about 6-7 miles north, of the city, I was looking up in the sky watching for the usual airline traffic from the airport. I used to live in Sioux Falls not too far from the airport and loved watching the planes and am well aware of all types of conventional air craft. The day was clear and sunny with just a few lines of whispy clouds. I was watching one jet fly over, east to west and then another came along also, east to west. A little higher in the sky, I noticed a bright round, white,light like a nickle at arms length and, I am estimating about 60 degrees off the horizon to the south and a little west of the city. It was very bright and appeared to be flashing though, it could have just been the reflection of the sun. At first I thought it was another plane but it didn't appear to be moving and didn't have a contrail as the other planes did. I asked my husband if he saw it and he said he did. It appeared to be moving in and out and above and below the clouds and would fade out from time to time. When the second plane came over, I noticed another light of the same type lower than the second plane and on the same course. It disappeared after about 5-10 seconds, the other one was still visible. I pulled to the side of the road to get a better look and try to get some pictures. It disappeared as we drove on, then, it reappeared and I pulled off onto the off ramp and drove down another highway looking for a gravel road so I could park and watch it. It was still in the same place in the sky and still doing the same things. By the time I had found a gravel road, it appeared to be higher up but still in the same place and barely visible, then it was just gone. I did notice another plane come from the airport, circle around at a lower altitude in the direction of the light but, don't think this pertained to it.