[unculture]

Much of the time on a typical London car journey is spent idling in traffic, so it’s not so easy to calculate.

[andylynch]

It is measured though! City hall & TRL track this. In outer London the average traffic speed overall is about 20mph. In inner London it’s about 12 and 8 in the centre. I don’t think the 20mph limit makes a real difference to journey times, but it does make the streets safer especially for people not in cars.