The current model Subaru already has this feature for it's "adaptive cruise control". It has 4 presets for distance it will follow the car in front.

It's obviously only appropriate on the freeway of course. On my commute I will have it set at the second shortest distance because it's frankly a little disconcerting when the person in front of you stops in a hurry. It does, however, mean that people merge in front of you... but somehow, with adaptive cruise, I no longer care.