[Here in Indonesia, we drive on the left lane of the road, which means the slowest vehicle will normally be on the left and the faster will be on the right.]
Let's imagine a three-lane toll road occupied by smoothly moving vehicles. The slowest vehicles, the trucks, the busses, and the trailers are there on the first lane. There will be some of those slowest vehicles that actually able to drive a little bit faster than the vehicle in front of them, and they take the chance, they will move to the second lane and pass over the vehicle on the first lane. The denser the road, it will be illogical for them to return to the first lane, for there will be another slower vehicles there. It will stay on the second lane instead.
On the second lane, same thing will happen again. Some vehicles will have their urge to pass over the vehicles on the front that though they are faster than the first lane but still felt too slow for some, then they take the third lane and stay there.
Those vehicles that occupy the third lane to pass over the vehicles on the second are not always the fastest in the traffic. Most of the times they don't have the speed capacity of the magnificent supercar, sometimes they are only of average speed, most of the times in a densely occupied road they are just a little bit faster than those vehicles on the first two lanes of the road.
"The maximum speed an individual vehicle can gain is inversely proportional to the density of the traffic."
"The denser the traffic, the closer the individual vehicle speed to the lowest speed of individual vehicle on the road."
An actual supercar with an actual superdriver might still be able to speed up in a dense traffic, turns itself right and left very tactfully and pass over all the slower vehicles on the road. But not without risk.
"The risk that an individual vehicle has to take is proportional to the maximum speed that need to be maintained and the density of the traffic."
"The higher the speed to be maintained by individual vehicles, the higher the risk that it has to take, and the denser the traffic, the higher the risk for the same speed."
What we learn is that a private vehicle based transportation system will perform worse as the density rises to a point where the speed of the entire road will be close to the slowest vehicle on the road.