[nkoren]

Reaction time is a relatively small factor in safety distance. Kinetic energy is a significantly larger factor. Currently, humans violate this as a matter of course, which is why we're so bloody unsafe. Automated vehicles will be far more punctilious, which at higher speeds means that the gains from reaction time will be more than offset by the respect for kinetic energy.

What's interesting is that automated vehicles, coordinating their interactions, should be able to accommodate additional traffic much more smoothly than human-driven vehicles. They will accommodate it by slowing down. Counter-intuitively, capacity is maximised at around 10 mph (16 km/h).

Here's a spreadsheet for doing those capacity calculations: https://docs.google.com/spreadsheets/d/1F3w5-hcopm2GBGz3UswN...

For humans, the reaction time is ~1 second. If you take that down to 0.1 seconds, you'll notice that the 10 mph capacity increases by about 1/3rd. That's not too shabby! But at a more acceptable 50 mph, the capacity increase is only 15% -- and that's if humans drove safely, which they don't. Actual capacity increase versus actual human driving behaviours, at highway speeds, can be expected to be nil.

Conclusion: automation will only bring capacity gains if roads get slower -- probably much slower.

[nkoren]

Actually, here's a more digestible illustration of the capacity gains from automation: https://docs.google.com/spreadsheets/d/e/2PACX-1vQ6ga2atFS69...