ABD guarantees that their trips (with a few exceptions) will not be cancelled within 70 days of departure.
If ABD cancels your trip prior to 70 days due to lack of bookings, they work with you very hard to work things out. If you want to switch to another trip, they will switch you, and will, within reason, pay for what it will cost to change your airfare, whether you booked the airfare through them or not.
In 2008, because ABD had just done a significant expansion right before the economy tanked, there were several trip cancellations that year. I was booked to do the Montana/Glacier trip, but it was cancelled (only 3 people signed up). There was an Alaska trip that left the day before the Montana trip was supposed to go, and they switched me to that trip, and paid to switch my airfare from Montana to Alaska (and I hadn't booked through them).
I know someone else who switched from a North America trip to an Italy trip, and while ABD paid all of the change fees for his airfare, they didn't pay the entire difference in cost between a domestic flight and a European flight (so, within reason). If you cancel, they'll refund the price of the ABD, and I do believe they reimbursed folks for their airfare, too, but I can't recall that one. I don't remember anyone actually cancelling and not re-scheduling. But it was a long time ago. There haven't been that many cancellations (that we're aware of) since then.
ABD has been known to go with relatively few people. My first ABD went with 14 people, my next with 15, and my last one had 18.
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