Abstract: A heavy road vehicle provided with a hybrid propulsion system includes a mechanical propulsion system and a hydraulic propulsion system. The mechanical propulsion system is connected to at least one traction wheel which is powered by an internal combustion engine via a mechanical drive train and a gearbox. The hydraulic propulsion system is connected to at least one other traction wheel including a hydraulic motor powered by a hydraulic pump unit. The hybrid propulsion unit further includes a control unit for at least controlling the hybrid propulsion unit, optionally also controlling the mechanical propulsion system. The hydraulic propulsion system further includes a pressure accumulator which is connected to the hydraulic motor in order to maintain a pressure in the hydraulic motor even if the hydraulic pump unit is switched off.