This is true when communication is transmitted particle-to-particle. However, before long, self-driving cars will likely be networked such that every car in a locale is communicating with each other, more like a quantum condensate. When the light turns red, every car will accelerate at the same moment, without a transmission delay along the string of cars. The same would go for merging traffic. All cars involved will likely be aware of the merging car's intent before it even gets on the ramp. It's possible that once you set your destination, current traffic will begin to optimize for your trip. I imagine that cars will be packed tighter in the left lane, and that lane change optimizations will take place as cars are intended to be added or removed.Even if they halved the safe driving distance, the gains would only be marginal, because each car needs to slow down to accommodate any new cars entering the freeway.