I had a space of 2+ seconds between the car in front and me, this individual stepped behind, passed to the left fast, accelerated and threw himself abruptly in front of me, making me stop so as not to hit him. I made a sign saying "no" with my head and the subject put out his hand and made the "ignorants sign". Then he speed up and was out of sight (he must have gone about 85-90 miles). This type of people is, in my opinion, a danger to themselves and to society.