This paper proposes a Cell Emission Approach for estimating CO2 emission in urban expressways. The estimation models rely on traffic flow data collected by traffic detectors. Two cell-based estimation models, called as the instantaneous model and the time slice model are proposed. The instantaneous model is one of the simplest methods for estimating CO2 emission. The time slice model is more accurate model than the instantaneous model because trajectories of each vehicle in a section of an expressway are considered. CO2 emission of the Hanshin Expressway in Japan is estimated by the proposed models. As the results of the estimation, gaps between the instantaneous model and the time slice model are confirmed. The qualitative characteristics of CO2 emission in the urban expressway section are presented by the visualization methods.