Cars do not belong in cities. A standard American sedan can comfortably hold 4+ adults w/ luggage, can travel in excess of 100 miles per hour, and can travel 300+ miles at a time without stopping to refuel. These are all great things if you are traveling long distances between cities. If you are going by yourself to pickup your dry cleaning, then cars are insanely over-engineered for the task. It’s like hammering in a nail with a diesel-powered pile driver. To achieve all these feats (high capacity, high speed, and long range driving), cars must be large and powered by fossil fuels. So when you get a few hundred (or thousand) cars squeezed onto narrow city streets, you are left with snarled traffic and stifling smog.
Even if you ignore the pollution, cars simply take up too much space. Next time you are stuck in traffic behind what seems like a million cars, try to imagine if all those cars where replaced by pedestrians or bike riders. Suddenly, the congestion is gone.