  The volume fractions of vacua in an eternally inflating multiverse are
described by a coarse-grain rate equation, which accounts for volume expansion
and vacuum transitions via bubble formation. We generalize the rate equation to
account for bubble collisions, including the possibility of classical
transitions. Classical transitions can modify the details of the hierarchical
structure among the volume fractions, with potential implications for the
staggering and Boltzmann-brain issues. Whether or not our vacuum is likely to
have been established by a classical transition depends on the detailed
relationships among transition rates in the landscape.
