Mud guards are used on larger vehicles for several reasons. One reason has to do with Newton's laws of motion. Newton stated that "an object in motion stays in motion." In this case, this means that when mud is carried around the tire, it is given enough momentum to continue in a straight path until something stops it. Without a mud guard, the mud and debris would continue and possibly cause a problem for the car behind. Trucks are higher than cars, so the level at which the mud would sling out from their tires would be likely to impact the following car's windshield. To prevent trucks and other larger vehicles from being a road hazard, we install mud flaps behind the tires. The mud and debris kicked up by the tires is then stopped and redirected harmlessly towards the ground.