The bus vs train vs flight decision framework I use for every leg of a multi-destination trip, because the right answer varies significantly.

Decision factors in rough priority order:

1. Time constraint: if a flight saves 8+ hours and you have limited trip time, the flight may be worth it even at higher cost. If you're slow traveling, time savings from a flight matter less.

2. Overnight option: if a bus or train runs overnight and arrives at a reasonable hour, it can replace both transport and accommodation. The combined value often makes it the clear winner over a flight.

3. Scenery value: some transport is itself part of the experience. The slow boat on the Mekong River, the train through the Scottish Highlands, the bus through the Croatian coast. Factor in the experience, not just the logistics.

4. Cost ratio: if the flight costs 3x the bus for a journey of under 4 hours, the bus is almost always right. If the flight costs 1.5x the bus for a 10-hour journey, the flight deserves consideration.

5. Airport overhead: add 3-4 hours of total airport time (getting there, security, boarding, ground transport at destination) to any flight journey for honest comparison. A 1-hour flight is often a 5-6 hour door-to-door journey.

The specific math I do: total door-to-door time and total cost for each option. Divide cost by time savings. If the more expensive option saves time at more than $15-20/hour, I'll consider it depending on my current mood and energy.