One thing to keep in mind is the average semi can gross 80,000lbs. Empty average weight is about 30,000. The savings is more for light products that can double stacked that won’t make the truck overweight.
The main problem with double stacking comes from the medium weight stuff that volume and weight wise can be double stacked with out putting the truck overweight. Then as the truck starts going down the road every bump they hit is bouncing those pallets making all that weight compress the lower pallet. Acceleration, braking, emergency braking, tight turns on exit ramps, etc all work to compress and make the load start leaning all over the place. When the pallets on top start leaning it makes them nearly impossible to unload. if you try to pick up the whole unit from the bottom pallet it will fall over. Most facilities don’t have low profile forklifts so they can’t pick the top pallet inside the truck or the top of the fork guard will hit the roof of the semi trailer. This means it is hand unloading and re-stacking time. This obviously takes a huge amount of time and money. Most times the drivers will hire a lumping service for this and expect to pay $100-$300 depending on the product and about a full days loss of time and wages.
That’s why in my first comment I said I’ve turned down tons of loads that i knew would be trouble. Considering when I drove i would average $300 a day driving and zero, zippy, nada, when sitting still waiting to be loaded or unloaded. It was financially beneficial to sit a day waiting for another load then to lose a full day plus lumper fees and having half your load refused because of damage. Not to mention when you back up to dock with your load leaning all over the place the dock foreman will usually tell you to pull back off the dock and wait till they have time for you to be blocking their dock for 8 hours when average unloading time takes 15-20 minutes.