I don't get that rule either, if they get pulled over doing 79km/h in a 50 zone, do the cops say good on you atleast you stuck to your speed limit, it really is stupid anywhere that allows you to go over 80km/h is a highway/motorway it's not like the car is harder to control at 90 than it is at 80 on a highway,
On that note have ever seen a p plater stick to that rule, I never did, I had to drive to blacktown 2 times a week for band practise I always went 100 on the m2