When a police officer writes a speeding ticket, is the limit an integral part of the charge? In other words, is the charge simply driving "x" km/h or is it "x" km/h in a "y" km/h zone?
A friend was driving on a road whose limit changes several times. He was, by his account, driving 82 in an 80 zone. An officer pulled him over and gave him a ticket for driving 82 in a 60 zone. The limit had changed from 60 to 80 nearly two kilometres earlier. It would appear the officer incorrectly identified the limit. So here's the question. Does the officer have to prove the limit was 60 for my friend to be convicted of driving 82 in a 60 or does he simply have to prove my friend was driving 82 so he can be convicted of driving 82 in what's really an 80? Yes, 82 in an 80 is technically speeding, but there isn't an officer in this province who'd pull a driver over for that.
If you can show that the speed limit was 80, the prosecutor can ask for the ticket to be amended to 82km/h in a 80km/h zone. The prosecutor also has the choice of asking for a directed verdict to dismiss the case if the trial has already begun, or if you are able to convince the prosecutor beforehand, he may withdraw the charge.
You could also try to argue that if the officer got the speed limit wrong, the vehicle's speed might also be inaccurate. 82 would still be considered valid based on radar, laser or speedometer reading, and theoretically, your friend could still be convicted for 82 in 80, unless there's other evidence to show that the radar/laser/speedometer reading of 82km/h is inaccurate.