The message
from Tony Bryer contains these words:

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

http://www.highwaycode.gov.uk/09.htm

In the units in which I learned to drive,

stopping distance (ft) = v(1+v/20) where v is the speed in mph

or v + v^2/20 thinking time approx 2/3 second and 0.6G ?

Perhaps it was the braking test I was thinking of, not thinking distance. :-)

I have now found 2 of my Highway Codes and the earlier one copyright
1978 still has the figures in feet. Thinking distance is conveniently is
one foot per mph and braking distance at 30 mph is 45 feet - 0.67% g.
The metric equivalents of 9m and 14m translate to 29.5 feet and 45.9
feet so the metric equivalents are a bit of a fudge, but then so
probably were the original figures.

--
Roger Chapman