Re: 1 hour of your time

On Mon, 13 Mar 2006 18:21:53 +0000,
Tony Raven <junk@xxxxxxxxxxxxxxxx> wrote:
davek wrote:

I don't buy that. What kind of timing are you talking about? Less than
a second? What's your stopping distance at 25mph? How long does it take
to cross a typical traffic-lit junciton at 25mph? How far are you from
the lights when they change?


Its not that simple as amber, stamp on the brakes. There is research on
the subject and there is a region leading up to the traffic lights known
as the dilemma zone (see for example "Yellow signal light timing and the
dilemma zone" Allen MJ, J Am Optom Assoc. 1995 Feb;66(2):77-8.) In that
zone, when the lights go amber you have to a) record that the lights
have changed, b) make an assessment of whether you are able to stop
safely at the line or whether braking will only leave you stopped in the
middle of the junction and c) take the action appropriate to that
assessment. A simple stimulus-response can be quite quick but where it
needs an cognitive evaluation step in between it can be quite slow. The
DfT describe the dilemma zone as "the distance over which it is
considered a change from green can present a driver with indecision
resulting in a high accident potential"


You can avoid this problem if you anticipate the "I'm committed" point
before you actually get there. The problem now boils down to being able
to correctly estimate where you can stop from. And if you do balls it up
completely and realise that there is no way you'll be able to stop in
time after you've started braking it's usually still possible to clear
the junction unless someone deliberately rams you from the other
direction.

TBH, on roads up to 50mph (in the dry) I've never really been conscious
of making a commit decision. The timings are such, and you get sufficient
experience of stopping a car from those speeds, that it becomes second
nature to judge whether you should stop or not. But for the occasional
lights on 60 and 70mph roads[1][2] I'm very aware of "4 seconds to the
lights, I'm not stopping" decision. And in the wet I will slow down
considerably so that my commit point will be 4s at 50mph (Using highway
code figures the stopping distance at 70mph is fractionally over 3s
driving at 70mph. And assuming your stopping distance is doubled you
actually won't quite stop in 4s driving distance from 50mph but in
practice if the roads are sufficiently wet that your stopping distance
would actually double I'd be doing 50 normally and slowing down some
more for the lights)

You have exactly the same problem on a bike, especially in the wet. Only
difference I make on my bike to driving a car is that on the bike I may
commit to stopping even though I can't stop behind the line if I will
still stop in a safe position and it's a large junction that I'm not
certain I can clear before the other direction goes to green.

[1] I'm referring here to the occasional lights that you can just drive
through at 70mph. They are very rare, you have to be able to see the
other controlled direction from sufficiently far away that you don't
need to start slowing anyway.

[2] I don't think I've ever brought a car from 70mph to a standstill
without modulating the brakes and/or going back onto the throttle in
order to stop exactly where I want to stop. Were I to try it and I got
within 50 feet of my target I'd be pretty chuffed. (This should be
something that would be trivial to set up in a simulator, get people
driving and used to how quickly the car will stop under various braking.
Then put them on a road with a red traffic light and tell them to start
hard braking when they can just stop. Once they've hit the brakes,
ignore any further modulation of the pedal and see where they really
stop - may be preset the braking level, then it can just be a button to
press)


Tim.

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

http://tjw.hn.org/ http://www.locofungus.btinternet.co.uk/
.

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