We're not really talking about that, we're talking about stop-and-go traffic, and smoothing out the stop-and-go portion through the use of an expanding and shrinking buffer in front. It will by necessity be larger and smaller at different times.

Additionally, when the gap is lessening because the car in front it stopped or at the slowest speed of the variable speed traffic, another vehicle inserting itself into this buffer can make you slow rapidly, thus negating the point of leaving a buffer to smooth out the variable traffic speed.