You only need reaction time factored in when the driver in front might act unpredictably. If the self driving cars pass information about the road ahead to cars behind, and let the car behind know what they are doing and planning to do the the required 'reaction time' is zero. Congestion is often caused by reaction times stacking up until a car has to completely stop. Then you have a stationary section of road that makes all the other cars that encounter it stop. If reaction time is zero, this removes that congestion, so more cars can drive on a section of road and not clog it. The same applies to intersections, roundabouts, on and off ramps on motorways. Just the simple act of negotiating with the cars around eliminates all of it.

Just think a little about what having a computer control the car would really mean, rather than thinking of them like robot people.