Level of detail

Most game have Level of detail is one way you can lower the poly count this is when the computer changes the high poly models with lower poly one when you are a long way away from that object this is used in almost every game so that they don't work the processor to much. To stop you from knowing that this it happening they have fade zones so closer you get to it, it will fade into the next model atleast 3 different models at the different levels but this can go up to 5.