Under the adverse driving conditions exception, how many additional hours can you drive?

If unexpected adverse driving conditions slow you down, you may drive up to 2 extra hours to complete what could have been driven in normal conditions. This means you could drive for up to 13 hours, which is 2 hours more than allowed under normal conditions. Adverse driving conditions mean things that you did not know about when you started your run, like snow, fog, or a shut-down of traffic due to a crash. Adverse driving conditions do not include situations that you should have known about, such as congested traffic during typical “rush hour” periods.
Even though you may drive 2 extra hours under this exception, you must not drive after the 14th consecutive hour after coming on duty.
Example: You begin to work at 7:00 a.m., start driving at 8:00 a.m., and drive 9 hours when you hit heavy fog at 5:00 p.m. The fog was not forecasted. At this point, the adverse driving conditions exception would allow you to drive for 4 more hours (2 hours to get to 11 and 2 extra hours due to the fog), taking you to 9:00 p.m. for a total of 13 hours of driving.
If, however, you come to work at 7:00 a.m., start driving at 12:00 p.m. and drive into fog at 5:00 p.m., you could still only drive until 9:00 p.m. for a total of 9 hours of driving. You would have to stop driving at 9:00 p.m. because you would have reached the 14 hour limit.