Regarding the Tenerife airport disaster of 1977, why did the departing aircraft have to turn around?

Back on March 27, 1977, KLM flight 4805 (a 747) collided with PanAm flight 1736 (also a 747) on the ground at the Los Rodeos airport at Tenerife, Canary Islands. It is the worst single disaster in aviation history in terms of lives lost. I recently watched a documentary about this incident and realized that one... show more Back on March 27, 1977, KLM flight 4805 (a 747) collided with PanAm flight 1736 (also a 747) on the ground at the Los Rodeos airport at Tenerife, Canary Islands. It is the worst single disaster in aviation history in terms of lives lost.

I recently watched a documentary about this incident and realized that one important fact was not mentioned. Although there is only one runway at Tenerife, aircraft can depart or arrive in two opposite directions. One is magnetic direction 120 (runway 12) and the other is magnetic direction 300 (runway 30).
The two aircraft in the collision were initially parked and waiting near the start of runway 12. The KLM was cleared to taxi down the entire length of runway 12, then turn around and wait for takeoff clearance on runway 30. The PanAm was cleared to also taxi down the runway 12, then turn off on one of the side roads to clear the way for the KLM to take off. Unfortunately, because of a thick fog, neither plane could see the other, the controllers in the tower could not see either plane, and the airport had no ground radar.
The collision occurred when the KLM began speeding down runway 30 to take off before the PanAm had turned off runway 12.
My question is why did these aircraft have to taxi the entire length of runway 12, turn around, then take off on runway 30? Since the aircraft were already waiting near the start of runway 12, why not use runway 12 to takeoff and thereby eliminate any turning around?
I don't recall them mentioning this important fact in the documentary.
3 answers 3