While I fully endorse this ideal, the reality of driving in high-traffic cities is that it's impossible. Every time you adjust to leave proper space, someone will come along and appropriate your safety buffer. When you slow again to allow enough space, the pattern repeats.
As others discussed throughout this thread, there are many situations where drivers violate the safe following distance rules. Aside from the lack of attention scenarios, I think many drivers take calculated risks to temporarily reduce their following distances while performing some other maneuver. They should incorporate this new idea of neurotic automatic-braking systems into this risk calculus, so they don't get caught in a situation they cannot handle.
While we have a mixture of different vintage cars (and drivers!), I am afraid we will remain living in interesting times...
I've learned the hard way not to expect anything from other drivers on the road.
Make sure your following distance is such that you have enough time to react to completely oblivious driving behaviour from other drivers.