You're right: when traffic is at its critical density, that's often what triggers the shift from the free-flow regime to the congested regime. But just because you make computers drive the cars -- even assuming they did it perfectly without randomly braking, which they don't -- that doesn't mean it eliminates that flipping between regimes. At best, it might get you a little bit more capacity before hitting that critical threshold, but eventually it's still going to, and then something -- a squirrel darting into the road, a sunbeam glinting off something the wrong way and momentarily confusing the AI, a bump that disturbs the car just enough to make it slow down a fraction of a MPH, etc. -- is going to trigger that shift to the congested regime anyway.