Whether cars are autonomous or manually driven, you still need adequate space between vehicles to allow for safe stopping distance. The only way a line of cars could all start and stop simultaneously would be if they were electronically linked together like a virtual train. In that case, what happens if the car in front has to brake suddenly for an unexpected obstacle?
True, but why can't they be linked together? Aircraft are linked together to a certain degree by TCAS which warns when another plane gets too close and orders avoiding action if needed. Isn't part of the reasoning behind autonomous cars to increase efficiency on our overcrowded roads? If the lead car has to brake or take avoiding action in a few milliseconds it can tell the cars behind to brake far quicker than a human can react which at best is ~215 milliseconds but that's the time it takes us to react to something, if we need to move from the accelerator to the brake it takes long hence the two second rule.
I don't think we have to worry though as it'll be decades before we reach that stage.
The problem with "progress" is that it's not always a leap forward.
To be fair, I can see the appeal of driveless cars for some. For me, I'd rather be in control of my own fate and while a driverless car may ultimately be safer, I trust myself. I may not be able to trust other drivers but that's the risk I take. I'm fine with it.
Quite often those most resistant to some of the changes are those who grew up without it, whilst those who grow up with a certain tech accept it as the norm. I suspect some where resistant to things which we accept as the norm in terms of safety feature in cars such as seat belts, ABS, Traction control and the newer techs such as cars braking themselves when they get too close to the car ahead. Don't get me wrong I can see the dangers of cars which are driven by computer, open to hacking to name but one potential danger. Perhaps in the future the happy medium is that if traffic is below a certain density you can take manual control and certain lanes on the motorways can be used for manual control.