Firstly I didn't "make stuff up", I expressed an opinion on both the 20mph and Khan's attitude to tradespeople. By your rationale, why stop at 20mph? Why not 15, or 10mph or have a person walking in front waving a flag? Ridiculous I know, but that's the level of the debate right now across the City. Since the introduction of competition for the Post Office, the explosion in online shopping which was accelerated during Covid, there is a significant increase in business traffic. So, in my opinion, unnecessary slowing of traffic across the City, creates delays and business costs. This can be highlighted by the changing of virtually all bus lanes from peak hours to 24/7 during Covid. I don't remember that being in Khan's manifesto or being consulted on that, though happy to be corrected.
As for pollution, given all our cars are supposed to be ULEZ compliant, the difference between 20mph and 30mph in suitable areas, can't be significant. Again, I have no problem with the 20mph in residential streets or outside schools, hospitals etc. I support the school streets programme too. I just don't see the argument on non residential roads where there are no obvious hazards.
So onto statistics. I've searched for various links on road traffic accident data and causation. The first link shows a decrease in fatalities specifically pedal cyclists (good news for me as I cycle way more than I drive). The big rise is in 'other road users' which is identified as e-scooters and e-bikes. Aside of that, not really revealing. See:-
Reported road casualties in Great Britain, provisional estimates: year ending June 2023 - GOV.UK (www.gov.uk)
Onto factors, another .gov.uk report opens with the headline "In the last 30 years Great Britain’s population has grown by 15 per cent (8.1 million people). Despite this growth in population, road fatalities have fallen by 68 per cent (3,655 deaths)". Obviously the majority of time is way before ULEZ, 20mph and Congestion Charge and is probably due to the increase in vehicle safety features and the cultural change in drive driving.
The obvious risk groups that stand out are young people and recent licence holders and OAP's. The conclusion is that better education can continue to drop RTA injuries and fatalities despite a rapidly growing population.
See:-
Factors affecting reported road casualties (publishing.service.gov.uk)
The data around 20mph to 30mph is varied and more theoretical than factual. According to the Royal Society for the Prevention of Accidents. One of the first studies of pedestrian injury and car impact speed3 found that
at 20mph there was a 2.5% chance of being fatally injured, compared to a 20% chance at 30mph, although this study is now regarded as having overestimated the risks. See:-
20 mph zones and speed limits factsheet Feb 2017 (rospa.com)
Hope this helps to shape opinions as to the merits and drawbacks of a blanket 20mph limit. I would add, I might disagree with your opinion, but I respect your right to express it.