In message , at 09:16:04 on Tue,
10 Jul 2018, Mike Bristow remarked:

But we are speaking *average* speeds, not the fastest possible.

Yes; but you said that trunk roads in London were so congested that
the average speed couldn't be more than 8mph. But a journey I made,
during the friday evening rush hour, had an average speed well in
excess of 8mph - and the fastest section - by far - was a trunk
road!

And that is a direct counter to your argument.

You appear not to be able to distinguish between the average speed of
all traffic (over all roads, at all times of day), and the speed you
averaged on one particular trip.
--
Roland Perry