Lets say for example that I clear you for take-off and you are sat on the threshold. By the time you spool up, roll then rotate and all wheels have left the runway, approximately 45 seconds will have passed. This is a minimum time and some airlines take considerably longer. If you don't believe me then time it and see.. .. .Let say for example that you are across the CAT 1 holding point but not quite in position on the threshold i.e. you are infringing the runway but still need to taxi round the corner and straighten up. If it is suddenly decided that we need you to vacate then we have to look at the options as to where we want you off. So, for the 27L departure configuration we have either 86 to the south, 86 to the north, 85 to the south and 85 to the north............ Following so far *winks*...... .Option 1, 27L vacate 86 to the south, firstly you need to power up to get moving then you need to taxi the required distance, then turn, then clear the runway. I would suggest at least 45 seconds even if expediting and you wouldn't want to expedite too quickly because ending up on the grass is embarassing. Block 86 to the south is fraught with problems though because we can't get you back to the hold by turning you left from 118 into 95 (see Air Pilot for reasons) and if there is something coming the other way from Terminal 4 then that route is blocked and we have a stand-off and the possibility of the runway not being clear.. .. .Option 2, 27L vacate 86 to the North. Same procedure as before except you will be turning directly into the teeth of other outbound traffic coming through 74. This is only a problem if the GMC controller is busy and hasn't handed the traffic to the departures controller. .Option 3, 27L vacate 85 to either the North or South. No major confliction problems BUT it is a greater distance to taxi thus increasing the time factor.. .. .okay, 27R for departure.... The first two available turn-offs are at 17 or 16 both of which are very tight reverse turns all of which take time.. .. .So, even if we have a mayday situation and need you off quickly, all these factors have to be taken into account by the controller (who at the time will also have the Mayday to worry about). These all take considerable time to achieve. I'm sure if there are any other Heathrow controllers reading this then they will concur with my comments.