[jraph]

> if you're really worried about collisions in the tunnels

Cars can fail, you have to leave enough room to allow cars behind to stop in time to avoid a (cascade) crash.

[aeternum]

This is mostly due to human attention and how the human vision system works.

[jraph]

There's also inertia, especially for cars that heavy. You can't go from 140 km/h to 0 instantaneously.

(yeah, taking 140 km/h, quite high for a car especially in tunnels like this, but anything slower would be slower than our slow trains and would be quite disappointing)