  Phase transition from a free-flow phase to a jammed phase is an important
feature of traffic networks. We study this transition in the case of a simple
square lattice network for different values of data posting rate $(\rho)$ by
introducing a parameter $p$ which selects a neighbour for onward data transfer
depending on queued traffic. For every $\rho$ there is a critical value of $p$
above which the system become jammed. The $\rho-p$ phase diagram shows some
interesting features. We also show that the average load diverges
logarithmically as $p$ approaches $p_c$ and the queue length distribution
exhibits exponential and algebraic nature in different regions of the phase
diagram.
