Aiming to enhance the level of monitoring of expressway status,a procedure of data preprocessing is presented,and structure of ETC transaction data within a calculation period is analyzed to establish a dual-level calculation model of travel speed under different levels of sample sizes.In order to improve credibility of velocity extraction under conditions of small samples,the ETC data of different enter-leave toll plazas pairs is used to extracting the travel speed of a road section.A velocity correction method is proposed bases on the speed reduction factor a and speed reliable weight θ coming from OD investigation.The model is verified by a special designed field experiments.The results show that the mean absolute error is 4.5 km/h,and the average relative error is about 6.5%.The mean square errors are 2.38 km/h and 3.39 km/h in peak and non-peak periods,respectively.It shows that stability of the model in peak periods is better than which in non-peak periods.However,the results cannot entirety reflect driving states.In future,other data can be applied to increase the accuracy and reliability of traffic running status on freeways.
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