Discription of Events: This semi passed us very aggressively while we were in the slow lane going the speed limit by tailgating with flashing headlights. Then, the semi quickly moved over into our lane when only about half of the truck past us, forcing us to swerve off of the Interstate and onto the shoulder/gravel. After this, we then noticed they were swerving right and left repeatedly, making other cars pass them quickly and obviously causing them concern as well.