How is it then that KLM manages to reach the end of the runway, turn 180 degrees, wait a period, begin to roll and nearly take-off whereas Pan Am only just manages to reach Taxiway Charlie 4 in the same time period?
When the KLM 747 was moving down the runway, it had relatively clear visibility, at least at the start and could move at normal or near normal taxiing speed.
When the PanAm 747 started taxiing, the fog settled over the runway, cutting visibility. Since the PanAm aircraft was preparing for a left turn, it also was moving at a much slower rate, cut even lower because of the fog. The fog was so thick, the PanAm crew could barely see the ground in front of them, let alone the C3 or C4 turnoff.