But there is also another factor, critical speed. That is the speed at which an otherwise completely docile unit will suddenly become unmanageable in the event of a change of conditions, whether brought about actively on the part of the driver, such as to avoid a sudden unforeseen hazard ahead or passively due to, say, a gust of wind. Below that critical speed, the chances of restoring stability are dependent on the experience of the driver but above that, even an experienced driver will stand no chance. The trouble is that the driver is also given no warning when the critical speed is reached and it is only in the event of an emergency situation that it becomes apparent. The critical speed varies form outfit to outfit and is dependent on a multitude of factors, too many to list here, but generally it is between 60 and 70mph. Over that speed, the outfit will appear to be quite stable until a hazard is encountered.