  The jam phases in a two-dimensional cellular automata model of traffic flow
are investigated by computer simulations. Two different types of the jam phases
are found. The spatially diagonal long-range correlation obeys the power law at
the low-density jam configurations. The diagonal correlation exponentially
decays at the high-density jam. The exponent of the short-range correlation in
the diagonal direction is introduced to define the transition between these two
phases. We also discuss the stability of the jams.
