Monopolizing a public thoroughfare for the duration it takes one elongated vehicle to pass another with a 3MPH delta should be illegal enough to result in a fine when it impedes the free flow of traffic IMO. For as much as claims of safety are thrown about, people should spend more time looking at how congestion causes and compounds accidents.
Maybe if they weren't over-regulated* and actually assigned the costs associated with blocking traffic, the trucking companies could come up with something as novel as a 'governor override budget'. I.e., "If you find yourself in a situation where you need to pass and it'd be best for everyone involved if you actually get it over with, you may do so. Just don't expect to spend the whole trip at the higher speeds because you only accrue so many override minutes per hour and you need to save them for complications that may arise. There are gas prices to consider, after all."
If we're going to insist on having humans behind the wheel, I assume it's because they have the capacity to reason. Ultimately, we need to incent them properly and then trust their judgement.
* Speed limits significantly below the 85th percentile of traffic == revenue. Also, penalizing motorists (who have, by definition, opted out of public transportation) by failing to build additional, needed lanes.