You'll also want to take into account the 300% increase in speed. Transportation isn't about static density it's about throughput.
Objects moving 3x faster can be 1/3 as dense and have the same overall throughput.
Objects moving 3x faster can be 1/3 as dense and have the same overall throughput.
(You can argue that people don't usually leave sufficient space to stop, but I will point out that self-driving cars, which prompted this discussion, are going to be following those basic safety rules)
Your complaint is that cars lower density as speed increases, which is true, but you're still looking at this too narrowly. Consider the bus.