2011 Headway calculation

[mustafa]

Dear peter,

We have solved 2011 module2 headway calculations and the same is attached to this mail. We need some clarifications regarding the following:
1.We assumed sighting distance as 10sec. for the speed of line. Is it correct or we need to assume it for headway speed?
we observed in some solved papers, they had taken it as nominal value of 200m. please clarify which is correct.
2. While calculating headway distance, we had taken the breaking distance with respect to line speed. Is it correct or shall we need to take it for headway speed?
3. Is it ok if we take decelerating and accelerating time in stopping headway calculations as same? or we need to follow the formula sqrt(2(O+L)/a)?, which we followed in this calculation.
4. Is it compulsory to assume some contingancy that should be subtracted from the required headway or shall we take direct value of obtained headway?
5. In the attached calculations, we didn't achived required stopping headway in either case. Is the calculation correct or we missed something?
please clarify as soon as possible.



Regards,
Mustafa

[Peter]

1. For UK applications, a 10 sec sighting time is an established norm. It should be at line speed as this is the worst case for a train approaching the signal. You will see a typical of 200m in some examples, but this a simplistic generalisation.

2. Braking must be at linespeed, again the worst case.

3. If, as is normally the case in the exam, acceleration and braking rates are specified and are the same, both times will be the same and given by v/a from the basic laws of motion.

4. Contingency is a value judgement. Talking about it demonstrates that you know there is an issue. Dealing with it will tell the examimer how much you understand. It does not matter if you deduct an allowance off of the quoted time and calculate to achieve it, or work out the numbers for the ideal case and then scale that to give the allowance, you get to he same point.

5. I have not yet been able to view your attachment so cannot comment on that at this time.

Hope 1 to 4 is helpful but ask if not.

Peter

[Peter]

For point 5, when you have considered the acceleration, why have you calculated the time taken for the train to accelerate over the distance O + L? How do you know what speed the train is doing at this point? You need to work out how long it will take the train to accelerate back to line speed. (Hint, given that the acceleration rate is the same as the deceleration rate, the time for both will be the same, and the distance travelled). I think this is the thing you are asking in point 3.

Peter

[PJW]

1. Whereas I would agree with Peter for the assessing of Minimum Reading Distance of a signal for signal sighting purposes that you must use the maximum line speed as the worst case, when assessing for headway considerations whether the driver of the 2nd train would see a cautionary aspect. then the headway speed is the one to use. However the 10sec is itself a somewhat arbitrary figure and the difference in the distance calculated at the two speeds is unlikely to be very significant.

4. You should read the wording of the question very carefully as it does subtly change from year to year. Sometimes the wording is such that it should be evident that they do NOT want you to make any contingency allowance at all and therefore it would be wrong to include.

Be aware that the examiners are trying to check whether you really have a full understanding rather than just be able to reproduce a learned methodology, so I recommend that you focus your study on developing this generic comprehension rather than worry unduly about particular points of detail for any particular year's question.

1. For UK applications, a 10 sec sighting time is an established norm. It should be at line speed as this is the worst case for a train approaching the signal. You will see a typical of 200m in some examples, but this a simplistic generalisation.

4. Contingency is a value judgement...
Peter

[PJW]

Since it is not long now until the exam,perhaps I should put on my "critical examiner" hat and tell you what I think they would say had this been your exam answer-

1. Don't waste your time writing out the question.
2. Don't use abbreviations such as BD without explaining.
3. Don't quote formulae without explaining their derivation, except when widely known using the common terminology (e.g. Newton's equations of motions)
4. Braking distance calculations- not all are needed so more wasting time. Answers not quoted to reasonable precision-train braking from 160kph is not within tolerance of a few centimetres, neither is it sensible to quote acceleration times in thousandths of a second
5. Not sure what you are calculating when attempting Headway for Stopping Trains- in fact it seems that you don't either.
6. Having decided that 3 aspects cannot achieve non-stop headway, what is the point of calculating the stopping headway for 3 aspects?

Overall haven't been too convincing that you know that you really know what you are doing; there are individual bits which are ok but answer wouldn't score highly.

Sorry if this seems harsh, but best you know this; actually it would not take much to considerably improve

[mustafa]

Since it is not long now until the exam,perhaps I should put on my "critical examiner" hat and tell you what I think they would say had this been your exam answer-

1. Don't waste your time writing out the question.
2. Don't use abbreviations such as BD without explaining.
3. Don't quote formulae without explaining their derivation, except when widely known using the common terminology (e.g. Newton's equations of motions)
4. Braking distance calculations- not all are needed so more wasting time. Answers not quoted to reasonable precision-train braking from 160kph is not within tolerance of a few centimetres, neither is it sensible to quote acceleration times in thousandths of a second
5. Not sure what you are calculating when attempting Headway for Stopping Trains- in fact it seems that you don't either.
6. Having decided that 3 aspects cannot achieve non-stop headway, what is the point of calculating the stopping headway for 3 aspects?

Overall haven't been too convincing that you know that you really know what you are doing; there are individual bits which are ok but answer wouldn't score highly.

Sorry if this seems harsh, but best you know this; actually it would not take much to considerably improve

hi PJW,
can you please send your comments in calculations and the purpose of posting is if both 4-aspect and 3-aspect headway is not within the required headway(even we didn't deduct contingency) then what is the solution?
clearly we know each step in the attached file but we did each possible step for searching required headway,
also I wanted to know whether the procedure we followed for solving the headway and breaking calculations is wrong?
if so please send the correct procedure in your opinion

[PJW]

I am on IRSE Convention at present, so unfortunately will not have opportunity to respond properly to this for several days; indeed I am getting quite a few other requests for last minute help in the run-up to the exam and have a backlog of such to deal with.

Your calculations for non-stop indicate that 3 aspect insufficient and 4 aspect sufficient. Therefore for stopping, just consider the 4 aspect case with the intention of showing that this provides the capacity.
I know that your figures for this suggest that it is not achieved and therefore there would be no solution- this may or may not be the case but there is certainly no value in proving that 3 aspects don't achieve as they are already ruled out by the non-stop calculations.

Primary problem is that you are not calculating the stopping headway correctly. You have based this on:
Ht4= Ht4(1) + decelerating time + dwell time + accelerating time.

Wearing my critical examiner hat, I'd say that I do not know what the terms you have used all mean, where this equation comes from etc. You haven't demonstrated to me that you understand what you are calculating or that you understand what the signalling constraint re stopping headway is. If you calculate the wrong thing then it is not surprising if the numbers don't work out.

I suggest you draw some diagrams and really try to think through what you have stated, what you wrote really meant and what you really should have written. This should both increase your understanding and allow you to find your error and correct.

Look at the other attempts on this website regarding the various different stopping headway calculations and the comments made upon them. Working through these should help and actually assist your understanding more than if I just corrected your calculations.

hi PJW,
can you please send your comments in calculations and the purpose of posting is if both 4-aspect and 3-aspect headway is not within the required headway(even we didn't deduct contingency) then what is the solution?
clearly we know each step in the attached file but we did each possible step for searching required headway,
also I wanted to know whether the procedure we followed for solving the headway and breaking calculations is wrong?
if so please send the correct procedure in your opinion

[Nagasri.Jonna]

Hi PJW,
we are attaching calculations for 2011 mod2 layout. please find and comment on the same.



Regards,
Nagasri.J

[PJW]

Overall these were quite good, certainly easy to read and easy to follow- however I think that they would have taken you far too long to do within exam conditions, so do need to abbreviate somewhat. Certainly some of what you included was never actually utilised- you do not need the braking distance for the speed limit over the turnouts for example.

Having determined the maximum spacing of signals that will give required headway, you should round DOWN not up!
Using your numbers I still don't quite agree re the "DGR figure", but that is minor. More important is to emphasise that this approach is not very popular with the examiners and if you utilise you MUST make it clear that you know the relevance of N=2. You have drawn diagram but you should have stated that unless can fit two min braking distances within the max headway distance that 3 aspect signals cannot meet the capacity requirements.

The question asked for:
a)Determine the minimum braking distances for the permissible speeds and braking characteristics of the traffic specified on layout 1.
b) Determine graphically or by calculation the theoretical best headway (without any allowances) at minimum signal spacing and the given speed

By this point you had already done a) or rather you had for the 160km/h but not for the 100km/h as far as I could see; it would have been good for you to have dawn attention to these figures in your answer.

I then got a little confused whether you were attempting to consider, having ruled out 3 aspects, the next most economical approach (i.e. 4 aspects spaced at 150% BD, note not BD+150%) or instead 4 aspects at minimum spacing. As I read the words of the question you did need to do the latter which in fact you did at the bottom of page 4. The info above this was actually relevant to the solution you need to adopt on your layout so was not superfluous but it would have helped had you been clearer when you were doing the exercise requested by the question paper and when you were justifying the solution to be used when actually placing signals on the layout. This presentation could also have been clearer that you were determining which of the maximum (i.e. capacity / overbraking) was the more onerous; again when you calculate the maximum do ensure you round DOWN rather than up.

For the stopping headway, it would have been better to have drawn your diagram on page 8 instead of the one on page 5. Also did not need to define all the symbols in Newton's equations of motion a second time around- you can't spare the time!
All was going well, if somewhat labouriously, until half way down page 6 where you compare the stopping train travelling 3084m with a non-stop train that you seem to claim covers 3716m (which happens to be the headway distance) in the same time. I rather lost you after that.
I also quibble with your last diagram as I can't rationalise the gradient of the crve with some of the annotations and also you seem to have marked some distances vertically although this is the time axis.

However overall you did show reasonable proficiency with the majority of the calculations and came up with a sensible range for placing the signals on the mainline. Given how close the exam now is I wouldn't worry about the finer points of the stopping calculations; better to spend the effort slimming down and speeding up your presentation and learning from some of the minor items above.
Similarly you need to leave yourself enough time to score the easy marks in all categories when doing the layout so I suggest that just showing how you'd start off the stopping calcs but then cutting your losses and moving on may well be a good policy.

Hi PJW,
we are attaching calculations for 2011 mod2 layout. please find and comment on the same.



Regards,
Nagasri.J

[PJW]

See attached comments