Visited the Eden Project, last week and watched the video of the building of their new exhibit the 'Seed'. This is a 70 ton granite sculpture, it was being transported on the back of a low-loader and it was held on with ratchet webbing straps.
The main thing to remember with straps is to get the correctly rated ones and have enough of them, also they will only be as strong as their mounting points in your vehicle.
NorthernToolsUK.com do a very good range with ratchet and webbing strength going up to 5-6000kg. Interestingly the weakest part of these straps is the metal hook on the end!