What is the average trailer bed height of an 18 wheeler ?

it can vary by a lot, it depends on what the load is, where it is going. Even trailers have different heights Part of a drivers job is to know exactly what height is and pay attention to the signs on bridges and overpasses. There are also flat bed trailers of different heights, some are called low boys they have a deck that is lower between the sets of wheels to accomodate different size loads.