[kibwen]

The footprint of the car matters. When cars get 5% longer, the same number of people in cars takes 5% more roadway, which adds up quickly, because the difference between smoothly-flowing traffic and jammed traffic is a fragile equilibrium dominated by breakpoints. Furthermore, heavier cars accelerate and decelerate slower than lighter cars, which has a compounding effect on decreasing overall throughput.

[bluGill]

That isn't true. Most of the space a car takes is empty as you need long distances between cars.

[kibwen]

No, the length you need between cars is variable and depends on the speed of traffic and the time it takes for a car to come to a stop. The longer a car is, the heavier it is (frames do not have negative weight), and the heavier it is, the longer the stopping distance is. Please don't bother commenting further on something you're so belligerently clueless about.