I note the reactive anger and the idea that fuel economy is ruined by doing 20 instead of 30 – and indeed, everyone should accept that, at a constant speed, 90% of ICE vehicles are most efficient at around 40mph.
but this isn’t about doing constant speeds, in an urban environment most driving includes a fair bit of stop and go – and traffic flows are generally more stabilised when drivers are forced to go slower – which is something that 20 urban limits very much help with.
so in real terms the fuel efficiency argument is a straw man when it comes to why 20 is plenty.

That fossil fuel efficiency argument put aside – within 7 years 80% of people will be in EVs – so not be driving ICE machines (that pollute more or less at any speed) at all… but with urban lower speed limits there will be fewer deaths and injuries from cars hitting people AND poisoning them with tail pipe gases.
EVs will be more efficient, the silly range argument will be destroyed by cars capable of doing 600+ miles on a single charge, and then we can concentrate much more pertinently on safer urban streets – where pedestrians have the rules on their side and cars cease to be tools for delusions of power to be acted out in the streets where people live.