>How about when merging, or changing lanes? The speed limit is not _always_ safe or practical to obey strictly.

In the case of overtaking, you just need to make sure you don't cut someone off in the overtaking lane. No need to go over the speed limit just because you're too impatient to wait for a proper gap in the traffic.

I've never seen a situation where you're merging onto a freeway and everyone in the right-most lane is speeding excessively. That would be a very dangerous situation, and if you yourself go significantly faster than the speed limit to merge you're just adding to the dangerous problem.