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The importance of real-time speed estimation is undoubtedly significant for traveler information and traffic management systems. Unfortunately, the most common form of traffic detector, the single loop detector, is incapable of providing speed measurements. Analyzing the weakness of mean effective vehicle length (MEVL), typical effective vehicle length (TEVL) was introduced instead of MEVL using in speed estimation. The TEVL is more robust than MEVL and has less difference in different detector stations. So the same value of TEVL can be set for all detector stations. Then a new method was proposed to reduce the error of speed estimation. This method used the 15th percentile vehicle passage time over the detector which was the typical vehicle passage time corresponding to TEVL. The algorithm of the proposed method is implemented and evaluated using the field data from urban expressways in Beijing. The results show that the proposed method has more excellent estimation accuracy than conventional method.