Um, no. Traffic jams are caused by too many cars on the road at the same time, not too many people. If all those people were in buses, the roads would be almost empty.
And it isn't population density that puts all those cars on the road, it's parking space density, which is pretty much the opposite of population density. Most cars on the road at any given time are traveling from one parking space to another, so the way to get rid of cars is the same as the way to get rid of pigeons: eliminate places for them to roost.