In this Mail article (thanks to FFDF for posting it), a “spokesperson” for Sadiq Khan claims the major cause of congestion in cities is roadworks. Not in my observation and experience. The major cause of congestion (and to paraphrase Joni Mitchell, I’ve looked at this from all sides now) is those weapons of mass distraction, danger and delay: traffic lights.
As for 20mph limits, I couldn’t agree more with the aim: to slow traffic in busy urban settings where people on foot, especially children, are about; but couldn’t disagree more with the means to achieve it, viz. telling people to drive by numbers when they should learn to drive by context, and be free to use their own judgement.