Last night we had rain followed by heavy snow, so the roads have been rather slick today with a lot of ice patches. I'm going south on I-35 around noon, driving under the speed limit (like most people) due to the slick conditions. This Jeep passes me, and there is a semi behind him. After the Jeep gets a little ways ahead of me, he slows down...way down...for no apparent reason. I would say, based on the speed I was driving, that he dropped his speed from about 70 to about 40 very quickly. The semi honked at the Jeep, and the guy in the Jeep hits the brakes and slows to probably about 25 (and this is on the rural interstate in the passing lane.) The semi starts fishtailing and weaving from lane to lane. The guy driving the Jeep sees this takes off flying...doing probably about 80 to 90. Good thing for me the semi was behind me or he would have hit me. Fortunately the semi regained control before anything bad could happen. I have seen a lot of stupid things out on the road, but this has to be the dumbest thing I have ever seen. You don't play games like this on the road, and you certainly don't do it with semis on icy roads.