Bill Marsh at Kluwer Mediation Blog says "Professor Bill Zartman refers to two criteria as defining 'ripeness' for mediation:
1. A mutually-hurting stalemate; and
2. An impending crisis."
That concisely expresses a lot of truth, IMHO. As does Bill Marsh's statement: "There are many modern day equivalents to the king’s threat of “hanging”. In a litigation context, the usual equivalent is a trial – although that is often not perceived (at least in advance) as a mutual suffering in quite the same way as both Lords being hanged! It does serve to make the point, however, that the context in which settlement discussions take place is a central aspect of the judgment calls which parties make."