I'm not exactly sure why it is but private car drivers will absolutely avoid driving behind a pickup carrying a cargo load of people. It doesn't matter how many passengers there are, even a lone rider is enough to put a wide buffer between the back of the pickup and the car directly behind it.
I've seen cars frantically signalling (if they're polite) then switching lanes as quickly as humanly possible just to clear the rear of such offending vehicles. That's pretty dangerous road behaviour, and I don't quite understand the rationale behind this traffical bizarreness. But monkey see, monkey do, I've picked up this habit too and dance the ol' lane-change switcheroo as safely as I can. I can only speculate on the following possible reasons:
1) Pickups have a speed limit -- slow -- and no one wants to get stuck behind one of those things for long because we all have places to see, things to go and people to do. Three minutes ago.
2) The passengers are not secure in their seats, and at any time can get flung out back, denting the car immediately following.
3) No one wants to make eye contact with a hoarde of tough-looking manual workers 'cos no one wants to see them for the people they are, but are happier to just let them work behind the scenes, out of sight, out of mind.
There's nothing we can do about 1) and 3), but at least for 2) there are going to be new safety regulations regarding their personal security being transported in pickups, so no more fling-dings to worry about any longer.
Oh, wait. There IS something we can do about 1) and 3)! The Traffic Act should make it compulsory that the only passengers allowed to be transported by pickup are babes in bikinis. That way, every male driver will absolutely want to plant himself behind a pickup, and that might bring the overall highway speed down to safer levels.