A heavy road vehicle comprising a traction system (1), said system comprising - a first traction wheel (2a) forming part of a first propulsion system (12) comprising a mechanical drive train including an internal combustion engine (4), - a second traction wheel (6a) forming part of a second propulsion system (13) comprising a hydraulic pump (8) for powering a hydraulic motor (7a), - a control unit (9) for controlling said second propulsion unit (13), wherein - said heavy road vehicle comprises a traction regulator (14) which may be shifted between two different driving modes whereof the first mode (N, NM, NH) defines that the first propulsion system (12) is used for traction and a second mode (H, HF, HR) which defines that only the second propulsion system (13) is used for traction of the vehicle and - said control unit (9) programmed to change the function of at least one control device (15) from its normal function in the first mode (N, NM, NH) to function so as to control to the travel direction and/or speed of the vehicle when the second mode (H, HF, HR) is selected.