The 80" width was for requirement of the clearance
lights......don`t know why the trailers used the 80" width though since they could go to 102" overall width and still be legal for the highways with clearance
lights and the trio ID lights.......If I remember right, there was a bit of a kerfuffle with the 1960 Pontiacs which were a bit over 80" wide but were exempt from the law requiring clearance lights for that width and the 1961`s were about a inch narrower.....any vehicles/trailers over 80" wide require clearance and trio lamps...Benny