Trucks only drive when cargo full

I see many trucks driving from the mine with just a tiny bit of capacity filled. Like 3 dirt of a haul truck.
Would like to see an edict or button to toggle a feature: "only start when full" or "wait till cargo full"

1. you can toggle off partially loaded trucks when you open the vehicles management menu (the little truck on top)
2. this is related to what the excavators have as their next load. if it's a different type then most times the truck will go dump what he holds right now.
   sometimes the truck will switch the excavator, but that's not 100%