I have noticed that your productivity increases as your drivers stats increase. I found that by setting Driver Training to long distance first, (all five levels) then change to ADR when complete from there I just went down the list until all the boxes were lit. Obviously as they progress down the lists their level and productivity increases and the higher price per load they can command.
I have a full complement of drivers (113) and a tad over £25m I own 46 large garages (17 of them without staff or trucks) and the money just pours in. The biggest problem is that your drivers seldom manage to backload and run back empty. Whereas I will just pick the highest value per km/mile load and transport it to its destination. As I do all this travelling the profits just continue to rise (as do the costs and wages bills) but it seems to work for me.
Unfortunately, now I find I cannot manage every garage I own (my goal was to obviously own and manage every garage in the game but now I find it isn't possible). What a let down, I now ask myself what was the point?