What is the direction of travel when a rock is flung up on the highway?

The actual direction, relative to the road, is very nearly straight up. A spot on the tire moves in a curve called a cycloid, and the horizontal component is always in the direction of the truck, or zero. Because of the position of the rear wheels on a truck, a rock from the rear wheel is likely to miss the back of the truck and go up for some distance, since it can be traveling at 70 mph. Of course, air resistance slows down its vertical speed pretty fast, but it has almost no horizontal velocity. And then you run into it at 70 mph.