  The formation and disappearance of spiral arms are studied by focusing on
jammed Keplerian gas in a coupled map lattice (CML) with a minimal set of
procedures for simulating diverse patterns in astronomical objects. The CML
shows that a spiral arm is a type of traffic jam, and its motion is governed by
both a gas inflow into and outflow from the jam. In particular, we present a
new approach to simply and directly evaluating the disappearance of spiral
arms, called "light-in and heavy-out". It is based on the gas flow rate
difference between the light inflow and heavy outflow leading to the
disappearance of traffic jams. Furthermore, we propose an approximate formula
for the remaining lifetime of spiral arms, which is immediately derived from
the "light-in and heavy-out" approach without calculating their pattern speeds
as in conventional differential rotation. The proposed formula is successfully
applied to the CML simulations.
