21. A driver who encounters adverse driving conditions during a trip may extend the hours of driving and service permitted under sections 9 and 10 and the hours of service permitted under sections 6 and 7 by no more than 2 hours to complete the trip if(1) the driver still takes the required 8 consecutive hours of rest;
(2) the driver removes all or part of the 2 daily hours of rest required under the second paragraph of section 13 that have not been taken; and
(3) the trip could have been completed under normal driving conditions without the removal.
Adverse driving conditions are adverse road or weather conditions that were not known to the driver or operator before the driver began driving or could not reasonably have been known to them.