On the other hand, failure to budget flight time properly has larger consequences than fuel budgeting: missed connections, stranded passengers, passengers needi

On the other hand, failure to budget flight time properly has larger consequences than fuel budgeting: missed connections, stranded passengers, passengers needing to purchase replacement tickets on different airlines at spot rates, etc. Accounting for the natural variance in flight times minimizes the costs of these inconveniences, presumably lowering costs to airlines and increasing satisfaction of most travellers. For myself, almost all of my connections are not within driving distance, and my schedule is usually unforgiving to overnight delays. I usually budget 2.5 h minimum for transfers, 3.5 minimum if entering the US because passport control has its own very large variance. Haven't had the inconvenience of a missed connection in the last 10 trips.