[Purposes] To solve the planning and design problem of road infrastructure in the future autonomous driving environment, this paper proposed a road impedance function model for mixed traffic flow applicable to the connected and autonomous driving environment. [Methods] First, the pros and cons of the traditional road impedance function were analyzed, and improvements were made. Second, the fundamental diagram model for mixed traffic flow in the connected and autonomous driving environment was built on the basis of driving behavior differences of different types of vehicles and the functional degradation of autonomous vehicles. The traffic capacity impact coefficient was applied to describe the relationship between the traffic capacity and the penetration rate of autonomous vehicles. Finally, the basic form of the mixed traffic flow impedance function was derived and verified by simulation. [Findings] 1) The road traffic capacity can be improved by introducing autonomous vehicles, and traffic flow capacity composed entirely of autonomous vehicles is more than twice that of traditional human-driven vehicles. 2) With an increase in the penetration rate of autonomous vehicles, the traffic capacity of mixed traffic flow increases, and they have a quadratic function relationship. 3) The road impedance function can effectively describe the variation of road travel time with traffic flow and penetration rate of autonomous vehicles. [Conclusions] The proposed mixed traffic flow road impedance function model under the connected and autonomous driving environment can effectively describe the variation of travel time on a road section.