At least you are aware that there is a danger to this behavior. As a motorcyclist, I can assure you that errant beetles and loose rocks kicked up by your own front tires are not limited to dirt roads or when you are following closely behind someone. They are a risk on any roadway, and many's the time I was grateful to be wearing a helmet with face shield down! I've also seen more than one dog jump out of a moving car through a rolled down window, at least one of those resulting in surgery to put the poor dog's hip back in place. But if it is a risk you are comfortable taking then good luck to you. For me, my dogs (currently I've not owned another breed other than GSDs) are not allowed to stick their heads out open windows, even if we are going slowly, not just because of the foreign object danger but also because it allows me to have some measure of control over them. They are also not allowed to come into the front seat, "car surf" (putting their paws on the center console between the two front seats), or stand while the vehicle is in motion. Again, it allows me to have control over them and it is safer for them in case of an unexpected incident. With a large dog so reactive that he is literally hitting the window to lunge and bark at people you may have to face the reality that you need to have better control over him for his own safety and yours than allowing him to stick his head out the window affords.