The objective of this study was to evaluate the concept of a heavy-duty tractor - motorized semi-trailer hybrid electric combination, which would have electric drive axles on the semi-trailer.The scope of the project included an analysis of the general concept of a power-driven semi-trailer, the positioning of the concept of the heavy-duty tractor - motorized semi-trailer hybrid electric combination in the general context of the technology, and the evaluation of the applicability of the concept for different duty cycles.Several transport activities were analyzed to determine specific duty cycles for heavy-duty vehicles: highway line haul and regional haul, construction haul, and off-highway hauling of raw materials, such as forestry transport with Class 8 and off-highway tractor-trailer combinations.The concept of a heavy-duty tractor - motorized semi-trailer hybrid electric combination was compared with a corresponding conventional combination (conventional tractor and semi-trailer), and with a combination of a parallel hybrid electric tractor with a conventional semi-trailer. PSAT simulations were conducted to evaluate the fuel efficiency and the influence on traction and stability was evaluated for each representative duty-cycle.The simulation results generally predict comparable fuel efficiencies for the heavy-duty tractor - motorized semi-trailer hybrid electric combination, and the correspondent parallel hybrid electric tractor with conventional semi-trailer combination. Depending on the duty-cycle, both hybrid versions would offer substantial fuel savings when compared to the conventional tractor-semi-trailer combination, such as up to 30% for off-highway hauling of raw materials.The analysis of traction performances showed evident gains both in tractive force and gradeability for the motorized semi-trailer. Moreover, the use of motorized axles on the semi-trailer would improve the longitudinal stability of the vehicle combination by decreasing the potential for sliding, especially in off-highway conditions.The evaluation of the impact on greenhouse gas emissions shows that the electric motorized semi-trailer concept would offer important annual reductions for all the analyzed transport activities, with generally attractive payback periods. Exemplifying for the off-highway hauling of raw materials using off-highway vehicles, GHG emissions reduction would be up to 108 tons/year per vehicle combination, with a payback period as low as 18 months, which is explained by very important quantitative fuel savings coupled with a relatively moderate increase in vehicle purchase costs. However, for highway and regional haul, the relative increase of the semi-trailer cost by a factor of three would make the concept unattractive.The concept of a heavy-duty tractor - motorized semi-trailer hybrid electric combination would offer the best dynamic, environmental, and economical advantages when implemented for off-highway transport of raw materials, such as forestry or mining transport, for which the relative cost increases are acceptable and payback periods are relatively short.