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I recall seeing a forums post by a traffic engineer once and they said part of the process of deciding speed limits is, once the actual optimal speed for the road segment has been determined, subtract ~10mph to pre-emptively avoid complaints by the elderly that the speed limit is too high.

That was regarding suburban road planning though, not sure if the same applies to freeways and other major arteries.

It's not the same. The police officer is a moving speed limit restriction so cars driving at the normal speed will catch up to it and slow down and they will continue banking up for as long as the car is driving slower than normal.

You wouldn't get that banking up with everybody following the limit.

It becomes harder to change though because if the culture is to go x over the speed limit, people will probably still go x over the new higher speed limit.

Exactly, yeah, this is the problem I have -- it's pragmatic in a certain way, but also contributing to a vicious spiral.