  Based of simulations of a stochastic three-phase traffic flow model, we
reveal that at a signalized city intersection under small link inflow rates at
which a vehicle queue developed during the red phase of light signal dissolves
fully during the green phase, i.e., no traffic gridlock should be expected,
nevertheless, traffic breakdown with the subsequent city gridlock occurs with
some probability after a random time delay. This traffic breakdown is initiated
by a first-order phase transition from free flow to synchronized flow occurring
upstream of the vehicle queue at light signal. The probability of traffic
breakdown at light signal is an increasing function of the link inflow rate and
duration of the red phase of light signal.
