The message
from John Rumm contains these words:

The ones in the highway code also seem to assume that you are driving a
Moris 1100 with crossply tyres and non servo assisted drum brakes.
(which since the published numbers have not changed since that may have
been the case is perhaps not surprising)

I don't think they have changed (other than the units) for a good bit
longer than that.

I was working on one project which involved the occasional trials
session over at the TRL research labs site. Some of the guys took the
opportunity to play with some of the measurement gear on their own cars.
They found that even the most mundane modern family cars could stop in
less than half the distance presented by the highway code.

I can't find a single copy of The Highway Code atm (I have 3 stretching
back over the later half of my driving experience) but ISTR that the
formula is 0.5 seconds thinking distance plus retardation at two thirds
g. I wouldn't doubt halving the thinking distance but retardation well
in excess of g from an ordinary road car seems a little suspect to me.
IIRC brake efficiency for cars only needs to be 50% so was it that that
was doubled?

--
Roger Chapman