WASHINGTON—A report from the U.S. Department of Transportation confirmed Thursday that at any one time, the average American city bus is occupied by at least four fully erect penises. "This persistent level of physical arousal remains a mystery, as we have found no link between these erections and either the attractiveness of fellow passengers or the intensity of vehicle vibrations," the report read in part, also noting that when one of the four engorged penises goes flaccid, another immediately hardens, as if to take its place. "Four is just a minimum, of course. During rush-hour traffic, there may be as many as 21 stiffened members on a single bus, while off-peak erection counts often hover around five or even six." The report's statistics do not include bus drivers, who are believed to remain constantly erect throughout their shifts.