My wife was driving in the left lane of I-85 northbound (I was her passenger) near the exit to Old Winder Highway. We were going about 75mph, in a 75mph speed limit area. The driver of the other vehicle sped up to about 85-90mph, then got into our lane and braked abruptly in front of our car, causing us to nearly rear-end her. (There were no other cars in front of her for half a mile, and no reason for her to brake.) She then got back into the right hand lane, made an angry gesture at us, and exited at the next exit.