[throwaway2245]

One major thoroughfare in London changed (if I remember right) from one-way in one direction, to one-way in the other direction, to two-way, in the space of a couple of years.

People got used to it very quickly, to the extent that they would not remember the change ever happened.

If you have a broad enough roads to have two-way streets everywhere, there's little motivation for it to change. [Edited to note: at the level of individual cities, OSM's directionality was often estimated to be 100% correct]

But, with some narrower one-way streets, there becomes a clear motivation to tweak things to improve traffic flows (and also tweak the surrounding two-way streets).