2 aspect headway

[mattslade222]

Hi,

I am just starting to get my head around headway calculations. I work mainly on metro that uses 2 aspect signalling. When I am calculating non-stopping headway, I can apply the approach I see in most 3 and 4 aspect examples.

See image attached of example.

What I am struggling to comprehend is doing the calculation like this would mean the train operator would have to drive at the red signal at full speed to within the sighting distance, which may be less than the braking distance. Therefore the operator would have to be certain that signal was going to clear for them. I can understand that in 3 or 4 aspect you are driving at a caution signal at full speed as once you pass the signal if it doesn't clear you can start to brake.

With this in mind, non-stop headway's for 2 aspect do I need to add the braking distance to the sighting distance in advance of the first signal?

I dont have access to any signalling designers at work so any insight is appreciated.

[Peter]

In your diagram, you appear to have all signals spaced at braking distance but you have called them two aspect signals.

In 2 aspect signalled areas, the block length is between two R/G signals (which would be very much greater than braking distance) and each R/G signal has a Y/G signal associated with it at braking distance. You would never have, as you have in your diagram, R preceded by G.

If the block length is such that signals need to be spaced near braking distance, 2 aspect signalling is not economic and 3 aspect is likely to be the solution.

[mattslade222]

Hi PJW,

Thanks for the fast response, so that's my first error I shouldn't have referred to this as 2-aspect. The example included is of shunt signals.

I guess in calculating headway in this scenario I need to make some assumptions about the operational practice: for example always start from rest?

[Peter]

Hi PJW,

Thanks for the fast response, so that's my first error I shouldn't have referred to this as 2-aspect. The example included is of shunt signals.

I guess in calculating headway in this scenario I need to make some assumptions about the operational practice: for example always start from rest?

Small point, the Peter that is PJW is differnt to the Peter that is Peter.

Certainly if you are talking about trains moving onto a running line and taking up capacity, you will need to think about the time it will take to get out, get ready and move out of the section. Since you would not normally have running moves from shunt signals, assuming trains would be starting from rest is likely to be valid.

It would be helpful if you draw out what the scenario is and ask specific questions as there are too many variables to work out exactly what you are struggling with.

Peter