Selasa, 16 April 2013

RE: [CanPassRail] PTC was Perceived Risk on Various Modes


22 seconds (including the 10% padding) has been standard long before I
started working in the industry. The time can be increased if there are
other factors, including long trucks operating slowly, high approach speeds
without advance pre-emption of adjacent traffic lights or "Prepare to Stop"
signs, number of tracks (which increase clearing time) etc. These are added
to the basic 22 seconds, rather than arbitrarily changing the basic warning
time. Safety regulators, including Transport Canada, are very concerned
about excessive signal operating time which is rightly considered a safety
hazard itself. This is because drivers are very impatient and if they
experience or expect signals operating too long they will race across
heedlessly as soon as they see or hear them, in order to avoid having to
wait. This happens even without excessive advance operation, and it
increases when the signals are perceived as operating too long in advance.
Transport Canada considers 35 seconds to be the maximum advance operating
time in most cases, but their inspectors can require gate delay if they feel
it warranted at particular locations. This will require the operating
sequence to start earlier. At most crossings the standard 22 seconds is more
than enough.

The amount of warning time was originally the result of some experiment but
is confirmed by calculation. The fact that something developed empirically
does not mean it is amateur, unreliable or crude. To the contrary, something
as safety-critical as the required operating time of safety devices must not
be developed without empirical methods. "Empirical" can be defined as (1)
Pertaining to or based on experience; (2) Pertaining to, derived from or
testable by observations made using the physical senses or using instruments
which extended the senses; (3) Verifiable by means of scientific
experimentation. What is derived empirically needs to be confirmed by
calculation and theory, just as something derived theoretically must be
confirmed by practical experience.

Don Thomas

[] On Behalf Of Knut
Sent: April-16-13 8:48 AM
Subject: Re: [CanPassRail] PTC was Perceived Risk on Various Modes

No Don,

Exit gates never crossed my mind since I have seen those for years and even
then only once or twice.

I was just picturing the worst case scenario which of course should be
covered off.

Something like this:
Vehicle approaches crossing
Lights start to flash - 20 second (or 22 second) interval starts
Roads are slippery, driver decides he can't stop safely in time - it's
actually not critical in his mind that he tries to stop since he won't be
hitting anything if he doesn't.
So "x" seconds later the front of the vehicle passes the crossing gate.
Say at this point the gates have just started to go down.
That's the 15 second point of the 20 second interval.
Vehicle is a standard tractor-trailer with a low bed trailer, crossing is in
poor shape which seems to be the norm now-a-days, at least here in Montreal
So driver slows to a crawl.
10 seconds later the gates are down completely, ie 5 seconds minimum before
the train arrives.
Worst case at this point (assuming the vehicle is still crossing without
incident, is that the rear end of the vehicle isn just past the crossing
So now there are 5 seconds to move that complete vehicle out of the path of
the oncoming train.

You mentioned that you have been working with crossings for 30 years, has
that interval always been 20 seconds as far as you remember?
With the much longer vehicles today than even 10 years ago, if nothing else,
the interval between the signal activation and start of the gates lowering
should be revisited and maybe made longer especially if the 20 second time
was initially determined empirically.

That's all I'm saying.



On 2013-04-16, at 1:25 AM, Don Thomas wrote:

> Knot, your conclusion that there are only 10 seconds to clear the crossing
> appears to be based on an assumption that there will be exit gates
> at the same time as the gates at the entrance to the crossing. These are
> common, but when they exist they operate later than the entrance gates. So
> any vehicle already on the crossing has 20 seconds to clear (actually 22
> seconds since there should always be a minimum of 10% extra time built
> The Canadian technical standard RTD 10 will give the actual requirements.
> Try or
> The standard doesn't discuss how the 20 seconds were determined. In 30
> working with crossings I never thought to inquire into the details.
> I gained the impression it was more or less empirical, and the 20 second
> standard was effectively reverse-engineered through trial and error.
> Don Thomas

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[Non-text portions of this message have been removed]

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