Let me way-in on that.
I grew up in Paris and the problem there is that the city is way too big for its own good. Every single mode of transportation is overcrowded: the subway, the trains, the streets, the circular belt ("peripherique"), the buses, the pedestrian/biking ways, the tramways.
This overcrowding comes from decades of political will to centralize everything in the country in Paris. The city was never designed to take that kind of traffic. The last major redesign of the city was by haussmann at the end of the 19th century. Since then, only minor adjustment has been make: subways, tramways, "les quais", circular belt. But they all contribute to bring more people in.
The only solution for Paris (and for French efficiency) is to push people, administration, businesses into other cities.