基金supported in part by (i) National Natural Science Foundation of China(NSFC), Nos. 70671100, 71072029, and Beijing Philosophy and Social Science, Research Center for Beijing Transportation Development for J.L. Zhang(ii) NSFC Research Fund Nos. 70971069 and 70772052, and the Fok Ying-Tong Education Foundation of China No. 121078, for Y.J. Li