To answer your question; yes you "should" obey speed limits. Does that mean I always obey speed limits? No I do not.. I'm often 10 or 15 over on the freeway especially if the flow of traffic is over by the same amount. In a neighborhood or school zone, absolutely I'm at or just under the limit.
That said, during rush hour all bets are off.. I know people going various speeds will be in every lane and I just deal with it..
Now if I'm out on the freeway with relatively light traffic and I run up on a person cruising the left lane that won't give way.. I will sometimes provide a free driving lesson. I start by flashing my high beams in an attempt to ensure the driver of the car ahead of me is aware that I'm behind them. If I'm satisfied that they have said awareness and they don't give way I will do one of two things.. 1) if at night I may elect to turn on my high beams and ride close to the left line trying to get my left beam right in their driver's side mirror while the right beam is illuminating their rear view mirror. This is a fun game and gets interesting responses..
. 2) if no one else is behind me I will pass on the right (using my turn signal of course) and pull back into the left lane in front of them. I will then ease off the accelerator to allow the car to coast and decelerate down to whatever ridiculous slow speed is sufficient to dislodge the offending car from the left lane. Of course I'm watching the lane behind so as to see any oncoming traffic. Lowest speed ever achieved was 10mph but only for a for a minute. This has never failed to achieve the desired result.. Namely pissing off the left lane cruiser.
I'm not sure any if drivers realize that that any car going any speed greater than they are going has right of way to the left lane on any road where there is a posted sign indicating slower traffic keep right. The sign is clear and does not mean slower traffic keep right unless you citizen believe you are entitled to enforce what you think is a reasonable limit. In fact, failure to give right away is against the law and left lane cruisers are law breakers. The idea being, you don't know why someone is going faster than you or the posted limit and the law requires you to give way. The other car could be in a legitimate emergency. I doubt they get all that from my lesson but in my self righteous and egotistical way I'm happy..
P.s. I never apply this lessons in my M5; only when in my F150 which I'm fully prepared to swap paint with.