Abstract: In order to research the pattern of the free flow speed(FFS) on the expressway, the measured FFS, the theoretical FFS and the 85 percentile speed and their correlation were analyzed statistically using the traffic data acquired by the loop vehicle detectors buried in the expressway in Shanghai. The attention was focused on the measure FFS, and the regression models between it and the radics of the horizontal curve, between it and the distance to the inlet or from the exit ramp, and between it and the traffic saturation degree. On this basis, a model was presented to estimate the FFS on the expressway without the need of the field data, providing a base for evaluating the service level of the expressway operation system and estimating its traffic flow capacity.

Key words: transportation engineering, free flow speed (FFS), statistical analysis, expressway, traffic saturation degree