AbstractDifferent grades of roads have different function goals during the process of planning and design. The extent of function realization has significant effects on the operational efficiency of a road network system. In accordance with the function description of urban expressways in the Code for Design of Urban Road Engineering (CJJ37—2012), this study presents the concept of urban expressway functional reliability. This concept is also considered as a quantitative and probability evaluation index of the achievement degree of expressway functions. The calculation method of expressway functional reliability is constructed by combining vehicle speed with the coefficient of headway variation. The similarities and differences between the level of service and the proposed function reliability are discussed. Three expressway sections in Zhengzhou city are taken as case studies. The independence between vehicle speed and headway variation coefficient is verified based on the real data collected through vid...