If you drove with zero distance between your bumper and the car in front of you, they will be unable to brake hard, because its trying to stop an extra ton of mass. you're fine sure, but they just ran over that kid crossing the road without looking. or just t-boned someone. and heaven forbid someone was tailgating you too.
autonomous cars can form trains because they can react orders of magnitude faster than a human can to suddenly changing driving conditions.
You seem to have taken the concept that the larger the differences in velocity the greater the damage and extrapolated more from it than you ought to have.
If you are two seconds behind, they could come to a near stop and you can crash into them at full speed.
If you drove with zero distance between the your bumper and the car in front of you, if they brake hard you will have no impact at all.
This is why it's safe for train cars to tailgate.
In my view, autonomous cars will likely one day form trains for higher traffic density and throughput while remaining safe.