what if the yellow lines are there to deter vehicles from parking there as well?
i would check to see if that strip is a designated fire route, by looking at that city's by-law
ex: http://www.toronto.ca/legdocs/municode/1184_880.pdf
i worked at 103 The Queensway, and they have Fire Route signs erected in many places but MLEOs couldn't issue the $100 fine, they could only do the $30 Private Parking fine... because City Council hadn't amended the by-law to designate that location as a Fire Route
having said that, i guess fire trucks would just rear-end vehicles out of the way if they needed to
your defence that the sign was not visible sounds like it might valid; make sure you go through the proper procedure of taking & submitting pictures; maybe even schedule a first-attendance meeting and show the crown what you have and avoid trial altogether