Information gladly given but safety requires avoiding unnecessary conversationthe first thing that struck me was, 'why is the print so small on that sign?' followed by, 'why is that sign so unnecessarily wordy?'
one can imagine a similar sign in another large city (i name no names) saying
Do not speak to driverbut not in san francisco! it is safety, not the driver herself, that objects to conversation. it is only 'unnecessary conversatoion' that should be avoided--don't hesitate to inform the driver of an emergency--though really, 'unnecessary' is unnecessary, since having a necessary statement to make to the driver would just force you to break the rules, anyway.
another highly unnecessary word merits mention: 'gladly.' information will not be given begrudgingly on the muni!
the driver isn't even 'shunning' unnecessary conversation or 'stopping' it or anything like that; just mildly avoiding it like a small, friendly child in the street, for safety's sake.
even consider the ordering of the sentence. the alternative, "safety requires avoiding unnecessary conversation, but information is gladly given," just really puts too much emphasis on the negative, doesn't it?
i'm still not sure it's exactly what i'd call 'evidence' for my friendly city theory, but it's definitely a good sign.