If you are looking for some magical do it all excuse that will get you off this ticket, there isn't one. You have to be far more sophisticated than that. You have to attack the officer's recall of the facts: what was the weather like, what color is the car, how many doors does it have? This is to establish that his memory is faulty. You have to create doubt. Ask him what does the sign say exactly?
Request disclosure and hope that you don't get a copy of the by-law that established the fire route on this property. Just because there's a sign, doesn't mean it's legal. There has to be a by-law creating a fire route. If they don't establish this, then they don't have a foundation to lay the charge.
You've got to think outside the box on this one, because on the face of it, you're guilty. You have to shift the arguments onto your terms not theirs.
Their argument: there's a sign, he was parked there, he's guilty.
Your argument: there's no by-law, he can't remember any details, I doubt he wrote the license plate correctly.
[on this last point, this is why I recommend on my site to never take a parking ticket in, always wait till you get a notice of impending conviction, then you can create doubt that the ticket was placed on your vehicle, it just happened to have your license plate written down.]
Also find out who wrote the ticket. Was it a cop, was it a by-law enforcement officer, or was it a private company that monitors parking on private property? The latter is unlikely to show up in court.
