We investigate the traffic flow volume data on the time dependent activity of Beijings urban road network.The couplings between the average flux and the fluctuations on individual links are shown to follow certain scaling laws and yield a wide variety of scaling exponents between 1/2 and 1.To quantitatively explain this interesting phenomenon,a non-stationary Poisson arriving model is proposed.The scaling property is interpreted as the result of the time-variation of the arriving rate of flux over the network,which nicely explicates the effect of aggregation windows,and provides a concise model for the dependence of scaling exponent on the exteal/inteal force ratio.