Canada's Memorandum of Understanding on Vehicle Weights and Dimensions (M.o.U.) now defines the configuration of eight vehicles, based on regulatory principles derived from assessment of vehicle dynamic performance against objective measures that correlate with accident involvement. This process screened out a wide range of trucks equipped with liftable axles that operate in central and eastern Canada at considerably higher gross weights than most of the trucks meeting the strict conditions of the M.o.U.The paper uses computer simulation to assess the stability and control characteristics of some straight trucks and tractor-semitrailer combinations with liftable axles against the same performance standards used as the basis for the M.o.U. It examines vehicle performance with liftable axles properly loaded, with them raised, and with alternative self-steering systems for the wheels of the liftable axle. It recommends some regulatory principles, and describes how one province has implemented these in its regulations. It then extends this into a discussion of approaches that might be considered by jurisdictions seeking to increase the allowable gross weight for heavy trucks.