Height charts

Okay, so you know those "height charts" people might put on the wall to mark how tall their kids were on X date? There's a very rough version of one of those on both doors of a city bus I took today. Not kid-scale, just adult-sized like "this is 5 feet, this is 5.5, this is 6." I have so many questions. Is it like an amusement park, you must be 4 feet tall to ride alone? Is it for reporting crimes: "the suspect who fled through the back door was no taller than 5'6"? Is it for if the bus floods? "Yeah we'll send a dispatcher out but only if it gets past the 5' mark, everyone can tread until then."