Depending on the flight, a "taxi" time long enough to trigger the three-hour rule is already a major waste. Remember that not every plane has its tanks fully to

Depending on the flight, a "taxi" time long enough to trigger the three-hour rule is already a major waste. Remember that not every plane has its tanks fully topped up every time it lands, and that every minute the plane spends with its engines running prior to actually taking off is a minute that it's burning fuel. I've been on flights that had to return to a gate to refuel because they spent long enough on the ground after pushing back that they could no longer reach the destination. In at least a couple of cases the long delay was predictable from conditions at the time boarding began, but the airline went ahead and literally burned money in pursuit of closing the door on time.