(16-04-2010, 11:42 AM)greensky52 Wrote: I have read that some one have made a calculation of 2008 Module 2 before, but some small errors in it, and he did not calculate headway and signal spacing for stopping trains. I attempt this calculation again, hope for your feedback!
My work is taking the attached powpoint as an example. It is my 1st work~
For first attempt it is quite good. Certainly neat and easy to read with good diagrams. Well laid out overall.
BUT you didn't explain enough, logically messy, some dodgy parts and some bits assembled in the wrong order to impress the examiner.
I think it shows that you have studied a range of relevant material and can reproduce it but perhaps you have not yet completley understood and therefore how to fit it together into a cohesive whole.
So looking at your answer
1. Explain the conversion from km/h into m/s
2. Explain how you calculated braking distance [it is at very foot of page but should be shown the first time you use it].
3. You quoted formula for headway distance without explanation or even defining the terms used within it, You had a very good diagram on page 2 which would have been better placed here to help explain it, but you also should have marked the headway distance an (assuming you are going to use it) the distance Green to Red as well as explaining "S" (the diagram effectively does the other terms).
4.For clarity you should have said that the values you obtained for 3-aspect and 4-aspect were the "best achievable" for that form of signalling with signals placed at minimum spacing.
5. Having done these calcs then you didn't need to do the bit at the bottom of page 1 re Green to Red distance. All you needed to do was to calculate was what headway was actually needed by the train service / specified requirements and then compare to to those values calculated for the two forms of signalling. In your calc you just presented the value of 1205 without explaininng its derivation.
6. If you do wish to use distance Gren to Red instead then you do need to explain. Don't just say that 1.5<N<2 means 4-aspects; justify by explaining the significance.
7. Also you initially said D4 = 0,5BD but later that D4=1307m which is not. You must be careful to distinguish between the "best possible" headway when signals are at minimum spacing, and the "headway" that results from signals are at a certain greater spacing. I think what you were trying (quite correctly) to do was establish what was the maximum spacing that would just provide the headway you were aiming for.
8. Page 3 starts well, but beware that this calculation of the additional time taken compared with a non-stop train is not taking any account of how a driver would drive their train given the aspects that they would encounter.
9. However it then seems as you don't really understand. There was no sense in calculating what the headway would be for 3-aspects; you have already rejected this option as it is not capable of delivering the non-stop requirement. Also you based your 4-aspect value on the "best possible" headway from page 1 rather than the headway relevant to the spacing you chose on page 2.
10. As a free standing diagram, page 4 was fine, but didn't seem to tie in well with the rest. You should have explained what you were using it to calculate and why. You should also have commented whether a train starting from rest at a station really needs to see a Green; in 3-aspect signalling then the answer is yes but in 4-aspect signal a Double Yellow is generally regarded as good enough to persuade the driver to accelerate as hard and for as long.
11. Continuing onto page 5, you try to calculate headway time- BUT you can't use that equation since the train is accelerating and therefore V is not a constant. To calculate t2 you need to know how much of the distance (3D+O+L) is left and therefore you must calculate distance d1 first.
Somehow you tried to get a value of t2 and d2 first!
12. So as an examiner I am left confused what value you are going to use for your signal spacing because on page 2 you seem to say 1307m whereas on pade 5 you seem to say 1966m and you don't have any summary to bring things together.
So as I said initially you seem to have got all the right bits and most of the separate elements are fine, but you don't completely convince me that you understand.
Pay a bit more attention to explanation, what values are minima, which are maxima. You are certainly on the right lines, but not quite there yet- not a bad first attempt though