This happened this summer in the area of Bald Creek Falls, in East Tennessee. Jeannie and I decided to drive to see a beautiful waterfall about an hour from our home.
Enroute, Jeannie (J) was feeling kind of horny and lowered her top and exposed her big, beautiful, tits. This got me horny as she played with them and rubbed her nipples.
I was driving on the Interstate in the right lane, the "slow" lane. I came up behind a truck doing under the speed limit, and asked J if she wanted a stranger to see those tits. She said sure. I got a lane over and slowly passed the truck and I slowed to match his speed as we reached the cab.
J's tits were fully exposed, and she started twisting the nipples as she was turned on by the anticipation of a random guy seeing her tits. We got even with the cab, and I stayed right beside him. J said he's not looking. Then she said, OMG, he's looking.
He was a 50ish man, and she said looked rugged and handsome. She said he smiled at her, and she waved. She let him see her rubbing her nipples and she thought he was jerking off. As we came to our exit, I slowed and got behind him and exited. J said they waved at each other as the view declined.
Eventually we reached the falls and since it was a weekday, no one was around. We parked on the side of the road as there was no parking lot. The falls were beautiful and hefty, as we have had a lot of rain recently.