The modern solution to that is RSTP, or somtimes, similar proprietary protocols. It also works on arbitrary networks, not just ring topologies. We use RSTP for failover in tunnel control and emergency systems here in Norway, usually arranged into hierarchial ring-like structures.
STP, like so many things, is best expressed in rhyming verse:
Algorhyme
I think that I shall never see
a graph more lovely than a tree.
A tree whose crucial property
is loop-free connectivity.
A tree that must be sure to span
so packet can reach every LAN.
First, the root must be selected.
By ID, it is elected.
Least-cost paths from root are traced.
In the tree, these paths are placed.
A mesh is made by folks like me,
then bridges find a spanning tree.
Radia Perlman
I was addressing the poster's point about SONET. Of course no recovery solution will survive if the graph is really split into parts.
In case of the tunnels there normally isn't a single point you can slash and bring down the whole system. The failover requirements, recovery times etc are dictated by national road authority and fire safety regulations.
