I was recently in San Francisco and noticed a plaque in the sidewalk
at a Muni stop with a poem that I would like to identify.  I don't
remember the exact location of the stop.  It was definitely for the F
line along the Embarcadero near the intersection with Sansome.  I
could provide a better description if anyone thinks it would help.

I assume that this is a poem, but it could be a quote from a larger work.  

The first line is "Once, in a great while".

Paraphrasing the rest:

You will see a sign that speaks to you so clearly that you do not
question its meaning or logic.  It is so strong that you follow it
wholeheartedly and others do not question you because of your
conviction.

At the end it is attributed "Icon, Mark Osaki".  I assume Mark Osaki
is the author and Icon is the title of the poem or larger work.

A complete answer to this question will include the full text of the
poem/quote (it's about 12-18 lines).

Bonus, tip-worthy information includes:
- More information about the author
- History of or guide to the quotes in the sidewalks of SF (I noticed
these in quite a few places)

There's some bio information on Mr Osaki on this page:
http://fullcirclejrnl.com/bios4.html?1050025803398