After noticing that a large number of my planned out of town trips failed to materialize, I crunched some numbers over planned vs. actual trips in the last three years. In practice, it seems like there's about a 60% chance that a trip I actively plan for will actually happen. There's no one thing that causes me to miss trips, just various different events: trip operator cancels, tour guide gets married, medical emergency, work issues, hurricane, etc. However once I've purchased a plane ticket the likelihood the trip will occur goes up to about 90%.

Based on this analysis, I plan to sign up for a trip to Ecuador in December. It's a bit closer to another planned trip to India later that month than I'd like, but there's only about a 30% chance that both of these will actually go.