There will not be car trains and autonomous cars will not be able to travel 'bumper to bumper' at highway speeds, the result would be pile-ups.
Car brakes vary - a lot - well over 60% between the best and worst.
Car A braking distance 305feet
Car B braking distance 485feet
Car A and B have to leave 485 - 305 = 180feet gap + reaction time.
Autonomous cars will have to leave up to 180feet gap + reaction time (small)
Car A braking distance 224feet
Car B braking distance 139feet
Car A and B have to leave 224 - 139 = 85feet gap + reaction time.
Autonomous cars will have to leave up to 85feet gap + reaction time (small) = more than 0.82 seconds gap + reaction time.
Reaction time for humans is actually already a small proportion of total stopping distance, especially at high speeds.
Humans already driver too close, they are supposed to leave a 2 second gap but they actually often leave less than 1 second gap.
Autonomous cars would need to leave at least a 1-second gap to account for different braking speeds, likely car manufacturers will leave a little bit more.
Conclusion: The gaps between autonomous cars will look no different than the gaps between cars driven on the roads today, often autonomous cars will leave a larger gap because they are programmed to drive safely 100% of the time.