Comment by mitthrowaway2

I'm going to double-down on this point because it does matter. In the real world some people are tailgating, but that does not cause traffic jams if and only if the road is running below peak capacity, where negative feedback self-corrects those traffic density deviations and gradually smooths everything back out. (Unless the tailgater causes a crash, of course).

But if the average following distance is such that the road is exactly at the peak of throughput, or any smaller, then any momentary dip into tighter following distances pushes the road it into a positive feedback operating mode, which triggers a traffic jam.

If you haven't seen it, here is a classic experiment demonstrating the effect: https://www.youtube.com/watch?v=Suugn-p5C1M

(But obviously the solution to traffic isn't 5 more lanes, it's viable alternatives to driving!)