I agree we should all hate driving but because it's a waste of so many people's time when mass transit exists and it's ludicrous that driving is so heavily subsidised and marketed that so many think it worthwhile. It is possible to drive safely, even if many (most?) drivers don't.

No variance is stated in the standard, if I recall correctly, so you could go as far under the limit as you like but never over, but going way too slow (if your vehicle can go faster) would fail a different point in the standard so in practical test, you probably want to stay within 15-20% unless there's good reason to slow further. Yes, hills are included, so you have to know how to cause regenerative or engine breaking, as well as mechanical, but this is rarely tested.