Sighting Distance

[NJK]

Dear Members,

For headway calculations we are taking sighting time 10sec as a assumption. For sighting distance calculation whether we have use maximum permissible speed or headway speed ?

Please clarify.

[Peter]

Dear Members,

For headway calculations we are taking sighting time 10sec as a assumption. For sighting distance calculation whether we have use maximum permissible speed or headway speed ?

Please clarify.

You would use maximum permissible speed since this is the speed at which a train could be approaching the signal. The headway speed is a theoretical speed that you use for design - ideally all of the trains will be running at linespeed!

If you are still unsure on this, please come back.

Peter

[NJK]

Dear Sir,

Thank you.

[reuben]

The term "sighting distance" can have different meanings in different contexts. Often it refers to the earliest opportunity to see /catch a brief glimpse of a signal / route indicator, and may be 1000m or more. Conversely, for a stationary train standing at a signal, it is the closest distance at which the driver can still see the aspect, despite it being in their peripheral vision, and that the long-range beam focus makes close-up viewing difficult - this distance is taken as 25 metres.

For the purpose of the headway calculation, "sighting distance" refers to a final period/ distance in which the driver should have an uninterrupted view of a static signal aspect, in order to properly take it in and make a decision as to what action to take. For the headway calculation, the driver of "train 2" must see a constant green aspect in this time / distance, so that they can confidently maintain full service speed, despite "train 1" being somewhere out there on the line ahead.

For the purpose of the IRSE exam, the headway-critical train service is usually specified to be running at less than permitted speed. In this case, the sighting distance allowed for in the headway calculation for this train service should be based on its service speed. The client specification asks for this service to run at this speed (very often, because the specific vehicles have a speed limitation lower than that on the line). So the fact that some other trains are allowed to run at a faster speed on the line is irrelevant for the drivers attempting to run a frequent service at the lower speed.

However, this is all rather pedantic since:

The difference between 10s at 120 or 140 kmh, say, is tiny!

The 10 s figure is already conservatively rounded-up from the actual figure of 8s!

And finally, if your requirement is for a sighting TIME not DISTANCE, then don't worry: Ignore any allowance for sighting in your basic headway equation (eg in "(2B + O + S + L)/V", let S=0), and simply add your required headway TIME allowance directly at the end. This avoids converting seconds-to-metres-and-back-to-seconds-again, and shows the examiner that you understand what you're doing.

Reuben

[kiran218]

Dear peter,

Sighting distance does depends on the speed of the train.

with regards,
kiran

[Jerry1237]

I wish, for discussion's sakes, to question the two absolutes in Reuben's post.

1) Paragraph 2 - "uninterrupted": not strictly true in UK mainline standards but for the purposes of the exam and as an ideal, perfectly correct.

2) Paragraph 6 - "8s": Again, for the exam, quoting an assumption that sighting distance will be taken as <<insert value and unit>> is highly recommended. 8s is a commonly used value for UK based on mainline standards but as Reuben states, some pessimism is normally included.

Jerry

[PJW]

The time it takes for a driver to become aware that there is a signal in their field of view and to distinguish the aspect and determine the appropriate response is what is meant here. This is a TIME and thus when converted to distance does depend upon the speed of the train.

I am aware that in India the term "sighting distance" means not only this time but also the time to brake to the subsequent red aspect, which of course is highly dependent upon train speed. However this is not the meaning of the term in the context of this post or this style headway calculation. Be careful not to become confused by the different meaning of terms by different people in different contexts. This is why in the exam you should ALWAYS EXPLAIN, not just assume that the examiner will interpret things as you intended.

Dear peter,

Sighting distance does depends on the speed of the train.

with regards,
kiran

[kiran218]

Thank you peter...

The time it takes for a driver to become aware that there is a signal in their field of view and to distinguish the aspect and determine the appropriate response is what is meant here. This is a TIME and thus when converted to distance does depend upon the speed of the train.

I am aware that in India the term "sighting distance" means not only this time but also the time to brake to the subsequent red aspect, which of course is highly dependent upon train speed. However this is not the meaning of the term in the context of this post or this style headway calculation. Be careful not to become confused by the different meaning of terms by different people in different contexts. This is why in the exam you should ALWAYS EXPLAIN, not just assume that the examiner will interpret things as you intended.

Dear peter,

Sighting distance does depends on the speed of the train.

with regards,
kiran