I would disagree slightly on that. 55mph was imposed due to dangerous road surfaces, and stays mainly only in place where roads are poor and unsuitable for higher speeds. Certainly in Florida and Georgia.
I am used to travelling at low speeds on London roads, who isn't. But there is a logical time and place for lower limits, and they are not every road in the borough. The irony is, the red routes which remain 30mph in most places are sometimes more dangerous and risky to pedestrians and other road users, than those roads the limit has been dropped on.
Then add to that the poor roll out of the limit, and the remaining flaws in the rules, such as the flashing matrix outside Brockley Cemetery which still flashes "SLOW 30"