B767-200
Air Traffic Control Fully Certified
Situational Awareness
ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy
General None Reported / Taken
Procedure; Human Factors
I was working Local Control; landing Runways 28; departing Runways 01.  Aircraft X; B762; was requesting Runway 28L for departure.  I departed Aircraft X off Runway 28L.  Aircraft X rotated before the intersection of Runway 01L.  I cleared Aircraft Y for take off on Runway 01L; anticipating that the aircraft would not rotate before the intersection; therefore projecting the flight paths would not intersect.  My local assist also agreed with this projection as we discussed it before I had cleared Aircraft Y. It appeared Aircraft Y started to rotate at the intersection of Runway 28L.  I think the two minute rule stating a projected flight path is rather vague.  I will not clear an aircraft for take off based on my 'projections'; but instead just wait two minutes regardless of what I anticipate aircraft performance to be.  Projections are just that and sometimes aircraft rotate at different points obviously based on numerous variables.
SFO Controller described a possible rule infraction.  Traffic cleared for take off on Runway 1L; rotated prior to the intersection of Runways 28L and 1L.  Immediately after aircraft had departed Runway 28L; the reporter noting the rule covering this operation is somewhat vague.