This work presents an SEM analysis framework of the interaction mechanism of city traffic congestion and related variables. Instead of the traditional transport economics theories, our work is based on the time–space economy perspective whereby the reasons for and solutions to traffic congestion are reconsidered. The main contribution of our work is to explore Downs Law and advance the following congestion paradox: No matter how traffic develops under the city agglomeration, road congestion will be relieved in the short term but will return to the original conditions in long term. We find that the traffic congestion cannot be avoided but that accessibility can be improved by providing a large-volume time–space tunnel. The conceptual framework and the paradox are empirically tested using the case of the Tokyo metropolitan area. We also recommend relevant authorities to improve the space–time conversion ability by establishing TOD mode.