Having done that, if you can show that the signage in place (your photos) doesn't match up to the required and approved drawings, you could try to contact the Crown's office with your convincing evidence that the road was not properly closed. Don't forget to let them know you'll be asking for the ticket info to get statements from the other three drivers who made the same judgement as you, and you might well make common cause with them. Not that such info will be easily forthcoming, but ultimately it is public. It would also be interesting to hear from the constable in the unmarked just what his/her innocent purpose was in entering the road, and to discover why an officer was actually stationed at, but not preventing cars from entering that closed road. Duty logs and assignment sheets should be on file and could be ordered as evidence, and you could conceivably call to court and ask each officer involved to explain their part in not correcting this danger to safety.
We have been assured that Crowns accord thoughtful attention to such presentations and, as they did with the former Ontario Attorney-General, would consider dropping a case they couldn't make as strongly as the Defence could if it came to Court.