A system is provided for the throttle control of a multi-combination vehicle operable by a driver where said vehicle includes at least two engines each engine operable by an independent throttle. One of the throttles is controlled by a standard throttle pedal, the other by a hand-controlled throttle. Under conditions of severe braking the second throttle is caused to be disregarded by the second engine that is forced to an idle position. Further, the second throttle can only become operable when the second throttle has been manually reset to a pre-determined position, generally the idle position. The system is appropriate in cases where the driver may be forced to apply emergency braking and is not able to control the hand throttle to ensure that the second engine does not continue to drive the vehicle. A further improvement to the invention lies in the fact that under emergency braking the transmission system coupled with the second engine, generally automatic, is forced to convert mode to protect the transmission from damage.