Sabtu, 20 April 2013

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


Knot, again,, the 20 seconds (actually 22 seconds) is the starting point.
Unless the laws of physics change, there is no need to change the 22 second
starting point. The 22 seconds is not based on length of vehicle, it is
based on the minimum size vehicle. This hasn't changed substantially in the
past 80 years or and isn't going to change in the next 80. Yes cars got huge
in the 1960s and 70s and then slimmed down after oil crises and pollution
concerns. But they are of the same order of magnitude. You note that there
are much longer vehicles on the road. Semitrailers and similar vehicles have
gotten larger and double or even triple trailers have become more common.
These do not require a change to the 22 second starting point. You build an
allowance for longer vehicles (and for other factors) onto the 22 seconds.
That can increase the required time above 22 seconds. It doesn't change the
22 seconds starting point. If you had had triple trailers 50 years ago, the
calculation to allow for them would have been the same as it is today.

I hope that lays to rest the issue of the 20 or 22 seconds, and how to
calculate additional time beyond that. It seems to me that the crux of your
argument should not be lengthening the initial 22 seconds, but lengthening
the time at all crossings to allow for longer trucks. That is not how
crossing warning times are established. They are based on the design vehicle
for the crossing. It is not considered good practice to design all crossings
for the worst case scenario. Doing so would result in excessive operation,
as defined by the regulators, which for most railways in Canada is Transport
Canada. Regulators and crossing specialists in other countries concur.
Excessive operation is considered to be a safety hazard and can result in
demands by regulators to reduce the signal operating time to what is
necessary under the circumstances, which includes vehicle length, speed,
gradients, number of tracks etc. Excessive operation is considered a threat
to safe railway operations (which under Canadian law includes the safety of
the public using or crossing the railway), because it tends to encourage
unsafe attempts by drivers or pedestrians to cross against the signals in
order to avoid what is perceived to be an unduly long delay to road traffic.
Railway safety inspectors will request railways to reduce excessive signal
operation. Rather than basing crossing and signal design solely on the worst
case scenario, the railway industry in Canada is mandated by government
policy to base such decisions on risk assessments. This does not mean adding
risk. Risk is something which can never be eliminated from any human
endeavour. (As an extreme but not facetious example, you cannot avoid the
risk of hazards throughout the day by never getting out of bed, because that
will bring the risk; ultimately the near certainty, of bedsores, starvation,
losing your job etc.) You can only manage risks by identifying, weighting
and mitigating them. Under risk assessment protocols sanctioned by the
regulator, a risk is not the same as a hazard. A garden rake lying tines up
on your lawn is not a risk, it is a hazard. Among the undesirable events
caused by this hazard are stepping on the tines and piercing your foot, or
causing the handle to fly up and hit you on the head. The risk is the
product of the probability and severity of each undesired event, which can
be further assessed by various outcomes.

It may be an oversimplification but it can be said that the risk of
excessive operation must be balanced against the risk of not providing for
the absolutely longest and slowest conceivable vehicle. You are entitled to
disagree, and if you want to change the process you would have to take the
issue up with the government, and in effect with the international bodies
that set consensus on the issue. If you are interested in professional
thinking on this subject you might like to begin with up the proceedings of
the biennial Global Level Crossing and Trespass Symposium, held last year in
London: I have been to some of
these conferences, which are attended by the top experts in the field (as
well as much lowlier practitioners like myself). I am not claiming authority
just because I have sat in meetings with the experts. I am just trying to
report what the leaders in my field have said.

To conclude, your assumption that longer trucks etc warrant increasing the
base warning time of 20 (22) seconds isn't unreasonable. However your
understanding of the methodology is incorrect. In addition, industry
(including government) best practices require appropriate risk management
rather than automatic universal protection against all worst case scenarios.


[] On Behalf Of Knut
Sent: April-17-13 11:51 AM
Subject: Re: [CanPassRail] PTC was Perceived Risk on Various Modes


I never suggested that an empirical method to determine the required warning
interval was was amateur, unreliable or crude.

What I am suggesting is that the warning interval that was determined many.
many years ago by whatever method may not be correct today with much, much
longer vehicles on the roads. One can't really predict what kind of vehicle
is going to pass at any given crossing at any point in time so they should
be designed to handle the worst case situation.
Maybe 20 seconds is fine - but it would be nice if whatever empirical method
used was know and one could verify that this time interval is still valid


On 2013-04-16, at 8:35 PM, Don Thomas wrote:

> 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
> without advance pre-emption of adjacent traffic lights or "Prepare to
> signs, number of tracks (which increase clearing time) etc. These are
> to the basic 22 seconds, rather than arbitrarily changing the basic
> 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
> it warranted at particular locations. This will require the operating
> sequence to start earlier. At most crossings the standard 22 seconds is
> than enough.
> The amount of warning time was originally the result of some experiment
> is confirmed by calculation. The fact that something developed empirically
> does not mean it is amateur, unreliable or crude. To the contrary,
> as safety-critical as the required operating time of safety devices must
> 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
> 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

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