Conceptually, you can imagine each semi going 25 in the right lane as a rolling lane closure that requires all traffic to merge left and pass like a compacted viscous fluid going now more slowly than whatever the slowest passing speed of the passing formerly right lane driver was. If that speed is 30, then it is the equivalent of a construction lane closure 1000ft in length with a 30mph limit in the lane closure. Now put one of these closures (slow trucks) every mile and it is only natural that speeds slow severely in even mild traffic with many 25mph trucks on a two lane highway and naturally grind to 25-30 if heavy traffic. It is the way traffic works.
When you have a truck passing at 26 a truck that was doing 25, the length of the effective construction lane closure grows to 3/4 of a mile while the speed obviously drops to 26 and traffic compresses further.
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