2000 Headway calculations (non-stopping)

[Douglas]

Hi All

I've attached the headway calcs that I presented to the London Study Group last week. Any thought then I'd be delighted to hear of them. One concern was that I had picked the wrong train length (passeger 200m) for the distance from Green to Red - I still think that that's the correct thing to do:

• because the maximum speed on the line can only be achieved by the passenger train; &
  

• even if I've gotten it wrong then a matter of 200m isn't going to kill anyone is it?
  

What do you think?

I'll be working on non-stopping headway for this module in a months time, or so. I'd be keen to see how anyone else handles this

Douglas

[PJW]

Indeed. Fundamentally headway is a TIME between trains; it only has an equivalent DISTANCE at a defined CONSTANT speed. If the freight train runs at a slower speed than the passenger, then eventually it will get caught up by the following train. On real railways this is exactly what happens when there is a mix of traffic- the art of timetabling is to make sure that the slower train has reached a loop into which it can be recessed (or a junction where it branches off in a different direction) before that occurs. Look at the timetabling of trains on the electic lines out from Liverpool St for example; the "all stations" leave shortly after a "limited stop, outer suburban" service so that they get to a station where they stop in a loop so that they can get overtaken by the next "limited stop" service that left the terminus considerably later. This is not constant speed headway, but a special case of stopping headway.

To summarise: if trains cannot run at the defined headway speed, then forget them for non-stop headway calcs (i.e. = following trains at same constant speed) but do make sure that they all trains have adequate stopping distance from their max speed (only have to worry explicitly if have worse braking rate as otherwise it is a no-brainer of course).

You are also right that 200m only makes 6 secs difference to the headway time achieved at that speed- you have put a sensible, but arbitary, 30sec contingency into the calcs but coud equally well have decided to put in 20sec or 40sec, so really of little consequence anyway.

Other comments:
#1. the sighting time strictly need only be at the HEADWAY speed. Indeed 10sec is a nominal value based on the fact that the Minimum
Reading Time of a signal (safety reaction time) is often assessed to be in range 8 - 10 sec but drivers may well react sooner than that (defensive driver training). SAFETY considerations mean that there must be braking at MAXIMUM PERMISSIBLE speed post-to-post; HEADWAY considerations mean that unrestricted aspect must be observed before driver decides that they must brake which will be on the approach to the signal, but this is relevant to the scheduled speed for which headway designed, [makes little real difference to the calcs, just the same as allowing a little more system reaction time],

#2. You are right that serial processor systems are muc slower reacting in giving "better aspects" in rear upon clearance of the relevant overlap track circuit than RRI so an additional couple of seconds could be sensible to make allowance for system reaction time. Indeed SSI deliberately makes track section inputs "slow to pick" as mitigation against "track bob" as well as being inherently slow due to need to scan inputs and then write the output later within the processing cycle,
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#3. Actually the thing that the examiners would probably least like about the answer is the lack of information why the ratio between the "Maximum Green to Red distance that just achieves the headway " and the "Minimum Service Braking Distance" being very close to 2 means that 4 aspect signalling is the solution. A diagram helps (but is difficult in Microsoft Word") but candidates must avoid quoting the number of N as if it were pre-ordained by the Almighty; you need to show that you understand that if signals were separated by the minimum SBD that it would not be ossible to achieve the required headway / so close to the limit that in reality it would not be tenable given the inevitale other constraints on signal sighting to avoid sighting issues / physical obstructions / good protection of junctions etc.

PJW

Douglas Wrote:One concern was that I had picked the wrong train length (passeger 200m) for the distance from Green to Red - I still think that that's the correct thing to do.....What do you think?

Douglas

[KrishJ]

While calculating the headway for a particular layout, we need the following inputs such as Line Speed, Dwell time (However N/A for Non-stopping trains), Platform Length, Train Length, Overlap distance for calculating CP, Acceleration, Service Brake, Emergency Brake, Drivers reaction time for placing the Sighting Points and with some assumptions. From this we can calculate the Headway in seconds.

Also if we know the Headway timing we can achieve by adjusting the signal positioning or introducing intermediate signals we can meet.

[PJW]

KrishJ Wrote:While calculating the headway for a particular layout, we need the following inputs such as Line Speed, Dwell time (However N/A for Non-stopping trains), Platform Length, Train Length, Overlap distance for calculating CP, Acceleration, Service Brake, Emergency Brake, Drivers reaction time for placing the Sighting Points and with some assumptions. From this we can calculate the Headway in seconds.

Also if we know the Headway timing we can achieve by adjusting the signal positioning or introducing intermediate signals we can meet.

Indeed- this is why it is important to declare practices and state assumptions. In the UK it is generally true that the overlap provided for "Mainline" is not directly based on emergency braking, whereas on "Metro" it is.
Another significant difference is that "Metro" signal positioning is primarily dictated by headway considerations and trains stopping in platforms, whereas signal spacing to ensure adequate service braking from first warning to stop signal is particularly important for "Mainline"- the signal spacing arranged due to braking gives implications for headway. Must go now- using free internet at departure gate in Singapore airport and approaching flight time!

PJW