80km/h In A 50km/h Zone

The officer in was utilizing a standing detector in a parking lot on the other side of an expressway overpass, where I was clocked at 80km/h in an area listed as 50. However there are several odd circumstances that I would like to note in regards to the area:

1) The overpass is outside of city limits, and is not in a built-up area. It is the only 50km/h overpass in the entire municipality outside of city limits. The very next exit on the expressway, has the same sort of overpass close to the same distance as this one, and it is listed at 80km/h. Both overpasses are in the same municipality. Is there some sort of defense for an unexpectedly zoned area or the reasonable expectation of this overpass being the same as others?

2) The standing detector was located in a parking lot past the base of the overpass, the exit of which is a few meters from the speed limit change up to 80km/h. Is there a minimum distance from any limit changes the officers need to maintain or defense in regards to expectations?

My record is in fairly good standing, with no incidents recorded within the past five years. I began to pull over the moment I noticed the officer pulling out of the parking lot, and was into the 80km/h area when I finally came to a stop, he did not need to turn on his lights. The ticket was written on July 17, and I applied for Early Resolution on July 29, awaiting mail notification of the date. There was no construction in the area, nor has there been a history of accidents on this stretch of roadway.

was there a sign that said 50 km/h?

1) No, not if it was properly signed (i.e speed limit signs stating 50 km/hr).

2) No.

Looking back over the area, there's a sign just outside of city limits, not too far from the incline to the overpass. There's a side road leading off between the speed limit sign and the overpass proper. It's a two lane road that turns into four around the point of inclining.