With a high enough density of automated vehicles cooperating cars can be packed much closer together at speed. The questions are 1) what's that level and can we ever get to it and 2) will we get people to agree to slave their cars to a cooperative scheme like that.
Such brigading is unlikely to have a meaningful impact on the capacity of non-limited-access roads (e.g., the street grid of Manhattan). It might help the capacity of highways leading into a city from suburbs or exurbs, but I don't think you'd even practically achieve a 2x throughput bonus at best, and any extra capacity you might add would be meaningless so long as the capacity of the surface streets at the destination is insufficient to handle the extra load.
Add in the extra load of deadheading of self-driving cars, and I strongly doubt that there is any meaningful traffic benefit and in fact there could well be extra traffic costs incurred... and that's before anyone accounts for the effects of induced demand!
They can hypothetically be packed closer together, however:
- What level of reliability is actually feasible for such a scheme? If you don’t have 5 9s, are you going to be causing catastrophic accidents? Is that level even enough? Even with near instantaneous reaction times and perfect judgement vs humans, you’re still limited by basic physics/stopping distance.
- cars still take up physical space, no matter how close the cars get you eventually run into a flow rate problem. With the addition of self driving cars deadheading to pickups, you’ll also have added traffic volumes from entirely empty vehicles.
