My dad's been making a living out of driving since he was old enough to drive: he's driven ambulances, army wagons, buses, coaches, and most recently HGVs - he used the redundancy money from a coach driving job to pay for his HGV licence. One of the first non-agency jobs he had was driving a bulk powder tanker, where he was paid a wage plus a percentage of the load. He quickly left that because of the constant pressure to get another load in. Slightly different to tippers, I know, but I suspect that a remuneration policy that rewards more loads is going to put negative pressure on driving standards.
I spoke to him about cyclists and tippers, and he mentioned that when he's in his car, he's wary when there's a tipper about. He also mentioned the higher percentage of owner-drivers in the tipper trade. Not only does this mean that there's added pressure on making every driving hour earn money (no load = no cash, whereas the empty time is calculated into, say, a supermarket truck's costings), but such vehicles are likely to be older, without the extra mirrors fitted to the newer supermarket trucks, which are contractually likely to be no more than a year or two old. And that's leaving aside the muck and increased wear and tear caused by moving, well, muck about.