Abstract: Through both the analysis of over 26,000 real measured data collected on Shanghai's Yan'an Viaduct and the comparison with Kerner's classic model, it is found that the flow-density figure shows that non-linear regional characters even exist in the low density section. In addition, a method to quantify the proportion of aggressive drivers is proposed by means of fitting different equilibrium functions to real measured data and then forming a piecewise function composed of the best-fit function in each density section. This piecewise equilibrium function is much closer to China's present traffic conditions. Keywords: Traffic Flow Models; Measured Data; Flow-density Figure; Piecewise Equilibrium Function ;Aggressive Drivers;