The control device (5) for operating the internal combustion engine (6) for driving a hydraulic pump (10) is connected to a shunt valve (16) in the hydraulic motor circuit (9) for driving the vehicle wheels so that this valve is returned to its normal position in which vehicle movement is prevented if the valve is in the shunt position when the control device is activated. The control device is a foot pedal. The front wheels (4a) are driven by a hydraulic motor (8) forming part of a closed hydraulic circuit which also includes the hydraulic pump.
