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 today.
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 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
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