It means what is the best headway which can be achieved given the precise values given. In reality one would generally make some "less than ideal" asssuumptions to closer reflect reality and these would be bound to detract from the figure that would be achieved in practice.
Hence in a case having a braking distance of for example 750m, 3 aspects could not ever be placed closer than that (excluding modified sequence that ruins the headway anyway) but in the real world often there is some conflict that prevents the precise position so therefore the real spacing will be larger than 750m, perhaps 760m, perhaps 779m, perhaps 805m etc. The headway for the line is governed by the longest spacing, so when calculating the headway for the line one might normally make a blanket estimate and hope that all signals would be placed in a defined range say "750m to 800m" and hence you have made an allowance of 50m and use 800m for the headway calculation. Should onyou end up having to space a particular signal at 805m on this line, then it would actually be outside the allowance made, but at least your initial estimate is far more accurate than if you had chosen the minimum value of 750m.
Similarly if the speed limit on the line is 60mph, then a driver must not exceed that value- the speed of the train is unlikely to be a cpeerfectly constant 60mph and thus will average slightly less, say 55mph. Indeed if there is a form of Automatic Train Protection that intervenes at 60mph, then the train can never ever xceed that value and possibly will be pveted from reaching it die to its safety tolerances, the driver will certainly always aim to keep below the speed of intervention- thus the average speed simply must be less than 60mph. This would be another allowance one might make if wanting a more realistic estimate of achievable headway.
A further thing is if the tmetable requires following trains at exactly 3 minutes, then the signalling must be designed to be better than that; otherwise when train 1 is 15 seconds late, train 2 will also get delayed by at least that amount; in fact if it gets cautionary aspect and has to brake it may actually lose significantly more time by the time it has been braking for most of a signal section and can see that the next signal is actually showing green. Therefore for a reliable 3 minute service, one might design the signalling for a 2.5 minute headway requirement, in order to have a "buffer" for perturbation recovery.
The question in this particular year was clear that it wanted pure unadultorated best theoretically calculated headway value, without any compensatory factor[b] appropriate for a more realistic estimate.
We know that this will mean that the number calculated will not be achievable in practice for reasons as described above, but the examiners were clarifying the basis on which they wanted the student to calculate
Kindly clarify, meaning for "without any allowances" in this question
Hence in a case having a braking distance of for example 750m, 3 aspects could not ever be placed closer than that (excluding modified sequence that ruins the headway anyway) but in the real world often there is some conflict that prevents the precise position so therefore the real spacing will be larger than 750m, perhaps 760m, perhaps 779m, perhaps 805m etc. The headway for the line is governed by the longest spacing, so when calculating the headway for the line one might normally make a blanket estimate and hope that all signals would be placed in a defined range say "750m to 800m" and hence you have made an allowance of 50m and use 800m for the headway calculation. Should onyou end up having to space a particular signal at 805m on this line, then it would actually be outside the allowance made, but at least your initial estimate is far more accurate than if you had chosen the minimum value of 750m.
Similarly if the speed limit on the line is 60mph, then a driver must not exceed that value- the speed of the train is unlikely to be a cpeerfectly constant 60mph and thus will average slightly less, say 55mph. Indeed if there is a form of Automatic Train Protection that intervenes at 60mph, then the train can never ever xceed that value and possibly will be pveted from reaching it die to its safety tolerances, the driver will certainly always aim to keep below the speed of intervention- thus the average speed simply must be less than 60mph. This would be another allowance one might make if wanting a more realistic estimate of achievable headway.
A further thing is if the tmetable requires following trains at exactly 3 minutes, then the signalling must be designed to be better than that; otherwise when train 1 is 15 seconds late, train 2 will also get delayed by at least that amount; in fact if it gets cautionary aspect and has to brake it may actually lose significantly more time by the time it has been braking for most of a signal section and can see that the next signal is actually showing green. Therefore for a reliable 3 minute service, one might design the signalling for a 2.5 minute headway requirement, in order to have a "buffer" for perturbation recovery.
The question in this particular year was clear that it wanted pure unadultorated best theoretically calculated headway value, without any compensatory factor[b] appropriate for a more realistic estimate.
We know that this will mean that the number calculated will not be achievable in practice for reasons as described above, but the examiners were clarifying the basis on which they wanted the student to calculate
(03-07-2012, 12:39 PM)NJK Wrote: Dear Members,at minimum signal spacing and the given speed for application on layout 1.
I have one doubt in 2011 Module main line practice layout question paper.
In question 1, part b given as
Determine graphically or by calculation the theoretical best headway [b](without any allowances)
Kindly clarify, meaning for "without any allowances" in this question
PJW