On Fri, 30 Jul 2010 12:24:43 +0100,
Chris Tolley wrote:

The thinking distance is merely the speed in mph expressed in feet. The
stopping distance is merely the speed in mph squared and divided by 20,
then expressed in feet.

vmph v ft thinking + v*v/20 stopping

20mph --- 20 ft thinking + 20*20/20 stopping ---- 40ft
30mph --- 30 ft thinking + 30*30/20 stopping ---- 75ft
40mph --- 40 ft thinking + 40*40/20 stopping ---- 120ft
..
70mph 70 70*70/20 ---- 315ft


Why is this surprising? This is just thinking time = 2/3 second and
CoF=2/3 and then rounded to easy numbers.

If you wanted it in mph - metres the formula would be

x mph - 3x/10 thinking distance and 3x^2/200 stopping distance

20mph - 6m thinking and 6m stopping - 12m overall
30mph - 9m thinking and 13.5m stopping - 22.5m overall
40mph - 12m thinking and 24m stopping - 36m overall
....
70mph - 21m thinking and 73.5m stopping - 94.5m overall


If you want it in kph - metres the formula would be

x kph - x/5 thinking distance and 6*x^2/1000

20kph - 4m thinking and 2.4m stopping
30kph - 6m thinking and 5.4m stopping
40kph - 8m thinking and 9.6m stopping
....
110kph - 22m thinking and 72.6m stopping



As for your Ford Anglia allegation, the Highway Code predates Ford
Anglias by several decades. The same figures were included in the 1946
HC, and may have been in versions before that; I can't be bothered to
look them up.

But you only need to pick a reasonable value for thinking time and CoF
and the rest is basic physics.

Thinking times from about .7s to about 1.5s is reasonable depending on
what you really mean by "alert driver" and CoF between about .5 and .8
for rubber on dry asphalt for a typical road-legal car at road-legal
speeds.

Obviously, this breaks down for high performance cars at very high
speeds. I doubt that any road-legal car generates signficant down force
at speeds much below about 100mph due to the dire effect it has on fuel
consumption.

Tim.


--
God said, "div D = rho, div B = 0, curl E = - @B/@t, curl H = J + @D/@t,"
and there was light.

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