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密度是判别交通流状态的一个重要参数，但是在实际中取得交通流密度却相当困难。本文提出了一种利用定点检测器数据（流量和时间平均速度）来获取交通流密度的方法。该方法首先利用了交通工程学的知识，将时间平均速度转化为空间平均速度，然后根据交通流理论中的流密速关系，求解交通流密度，并指出空间平均速度的变异系数对密度的影响很小。该方法直接利用定点检测器数据，对于布设较多检测器的城市快速道路具有很高的可操作性和适应性。

Estimation of Traffic Density on Urban Freeways

Traffic density is one of the most important parameters to identify traffic state, but it is very difficult to be acquired in real world. This paper has proposed a method to estimate traffic density using the output (traffic flow and time mean speed) of fixed traffic detectors. The method firstly converts time mean speed to space mean speed by introducing coefficient of variation and standard error of space speed and then, acquires traffic density through the relationship between traffic flow, density , and space mean speed in traffic flow theory. Furthermore, the paper has indicated that traffic density is insensitive to the coefficient of variation of space speed. In the end, the method has been applied to Shanghai inner elevated ring freeway and traffic density can be reasonably estimated.In addition fitting curves among traffic flow, density and space mean speed are acquired. The results are similar to classic traffic flow theory. The method can estimate traffic density directly based on the outputs of fixed detectors, especially for urban freeways with lots of traffic detectors which are laid with mutually short distance. It is proved that the method is very simple and applicable to urban freeways.