Colour Light Signals - Light Order

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Inversnecky

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Apologies if this has been asked before (couldn’t find anything on a search).

I was wondering why the order of colour lights for railway signals is opposite to that of traffic light on roads?
 
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Gloster

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If you mean the order top to bottom, I believe that red is placed to be at the driver’s eye-level as it is the most important aspect. Yellow is above as it is next most important and then green goes at the top. When a fourth aspect was added it was put above the green so as to reduce problems caused by two adjacent yellows seeming to merge (*). I believe there are psychological or optical reasons why the lights go up, rather than down. There have been other arrangements, both at home and abroad.

* - It might seem to be playing safe by having a situation where a driver might mistake a double yellow for single. However, there is a risk of them getting into the habit of thinking, “This one’s always a double yellow,” when today it isn’t.
 

Supercoss

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Yellow Green Yellow Red. Top to bottom
This layout is to ensure red aspect is not obscured By snow sitting on ‘ hood’ of aspect below As no hood below bottom aspect to allow snow Build up ! Ground mounted main aspects have red at top but no hood below .

hood is like visor / brow or ‘ mud guard’ shaped fitment above aspect
 
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Ianno87

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The red must always be the one nearest the driver's eye level. That's why ground-mounted signals are "upside down".

As others have said, the yellow aspects must also be separated to avoid being mis-read.
 

21C101

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If you mean the order top to bottom, I believe that red is placed to be at the driver’s eye-level as it is the most important aspect. Yellow is above as it is next most important and then green goes at the top. When a fourth aspect was added it was put above the green so as to reduce problems caused by two adjacent yellows seeming to merge (*). I believe there are psychological or optical reasons why the lights go up, rather than down. There have been other arrangements, both at home and abroad.

* - It might seem to be playing safe by having a situation where a driver might mistake a double yellow for single. However, there is a risk of them getting into the habit of thinking, “This one’s always a double yellow,” when today it isn’t.
When 4 aspect colour light signals were first installed in the 1920s they were in two forms, neither of which are the current arrangement.

Either from top, Green, Yellow, Red, Yellow.

Or in a cluster with the two yellows vertically above each other and the red and green on either side on a circular plate.

The clusters soon fell out of favour because the red not being inline with the other signals was feared to make SPADs due to misreading more likely (something seemingly forgotten by the time SN109 was erected at Ladbroke Grove).

The original vertical practice of the first yellow being below the red signal with the second being above between red and green fell out of favour as it was possible for snow to build up on the hood of the first yellow lens, obstructing the red. By moving the red to the bottom this could not happen.


There is a picture of some cluster signals at the link below. They are "multi headed monsters" as the original colour light practice at junctions was to replicate semaphore practice and have a separate head for each permissible route option. This was discontinued in favour of "feathers" in quite short order because it meant that red aspect(s) were shown at signals which could be passed.

For the same reason the original subsidiary signal practice of a miniature red/green signal was soon discontinued.


 
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Inversnecky

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Thanks for the replies: the two notions of red at the bottom being:

1) closest to driver’s eye; and
2) less likely to be obscured by snow

seem to be the only reasons I’ve indeed come across.

Which begs the question, why aren’t road traffic signals as sensibly ordered?!
 

Ianno87

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Thanks for the replies: the two notions of red at the bottom being:

1) closest to driver’s eye; and
2) less likely to be obscured by snow

seem to be the only reasons I’ve indeed come across.

Which begs the question, why aren’t road traffic signals as sensibly ordered?!

Presumably, for road traffic signals, red at the top makes the signal more visible over a line of traffic.
 

edwin_m

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Signals are occasionally mounted horizontally, particularly for mid-platform signals under a canopy, with the red nearest the platform edge. However these can only be three-aspect as double yellow has to be displayed vertically - two yellows horizontally is a "splitting distant" for a junction signal showing danger.

I believe road traffic lights have to be displayed vertically and in the correct order, because colour-blind drivers distinguish the indications by position. People with colour-blindness aren't allowed to be train drivers.
 

Taunton

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Many earlier signals were Searchlight, which has of course made a reappearance with current, slightly different technology. These use a single light (so no hood/snow issues) and a mechanically-moving slide with red, yellow and green filters for the aspects. In the days of simple electrics, the connection from the signalbox was done with a single pair of wires for setting the aspect. The colour aspects, inside the signal, were mounted in a U-shaped slide, with red at the bottom, and yellow and green on each side. Positive polarity swung it one way, to the green. Negative polarity swung it the other way, to the yellow. No current and the weighted slide fell to the mid position and displayed red. Bulb power was provided by batteries, and a lot of the ones on open line were approach-lit to save current.

The LNER installed them quite extensively in the 1930s. The GWR did things differently (of course) and although they used searchlight units, they were just 2-aspect, replicating exactly what semaphore signals would show at night, with two units, well separated, mounted one on top of the other for stop signal over distant. Searchlights came from the USA where they were (and still are) very widespread, and the UK ones were initially imports and then produced by a licensee. Are there any mechanical ones still left? When did the last one go?

The ones on the GE main line lasted just into TPWS times, and the connection to this was done directly off the mechanical mechanism. Although the current polarity changed immediately, searchlights could take a few seconds to move the slide from one side to the other, yellow to green, passing the red briefly on the way, which GE drivers were just used to. But if the signal was just changing from yellow to green as the train passed, there was a second of red, and the train gets a TPWS trip. This was of course sorted out in short order, but one wonders if the original TPWS designers had any idea how they worked, or why they used the physical mechanism instead of the wiring.
 

py_megapixel

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I believe road traffic lights have to be displayed vertically and in the correct order, because colour-blind drivers distinguish the indications by position. People with colour-blindness aren't allowed to be train drivers.
Incidentally the PRM TSI requires that door open and close buttons be fitted in a specific order. Interesting, as they aren't usually colour coded, but I guess if you had some kind of visual impairment which made it difficult to distinguish "<|>" from ">|<" then it would help. In the UK so many units have a derogation from it that it's near useless, however.

Similarly I believe pedestrian crossings in the UK have to place the "stop" legend (i.e. the red man) closest to the top left, and the "walk" one (i.e. the green man) closest to the bottom right.
 

QueensCurve

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The red must always be the one nearest the driver's eye level. That's why ground-mounted signals are "upside down
What is bugging me now is that the only ground mounted ones I can think of are at Lancaster (P4 and Up Through) and I don't recall having noticed them having the red at the top.
The clusters soon fell out of favour because the red not being inline with the other signals was feared to make SPADs due to misreading more likely (something seemingly forgotten by the time SN109 was erected at Ladbroke Grove).
The unusual "L" shape of SN109 is a human factors issue that may have been underestimated in the case of the 1999 collision there.
Signals are occasionally mounted horizontally, particularly for mid-platform signals under a canopy, with the red nearest the platform edge.
The only place I can think of with this is Birmingham New St where there also seems to be an approach control on the signals.
 

21C101

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What is bugging me now is that the only ground mounted ones I can think of are at Lancaster (P4 and Up Through) and I don't recall having noticed them having the red at the top.

The unusual "L" shape of SN109 is a human factors issue that may have been underestimated in the case of the 1999 collision there.

The only place I can think of with this is Birmingham New St where there also seems to be an approach control on the signals.
City thameslink has horizontal signals from memory, possibly had rather than has.

Pretty sure that Farringdon has a ground mounted signal for departing southbound frpm plat 4. Cant remember where red was.

There are lots of miniature ground or low mounted signals in the Thameslink core tunnels.
 

jfollows

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Incidentally the PRM TSI requires that door open and close buttons be fitted in a specific order. Interesting, as they aren't usually colour coded, but I guess if you had some kind of visual impairment which made it difficult to distinguish "<|>" from ">|<" then it would help. In the UK so many units have a derogation from it that it's near useless, however.

Similarly I believe pedestrian crossings in the UK have to place the "stop" legend (i.e. the red man) closest to the top left, and the "walk" one (i.e. the green man) closest to the bottom right.
I don't have any kind of visual impairment of which I'm aware, nor any kind of related mental impairment, but I find it hard to understand which button is which for door open/close, the "<|>" and ">|<" symbols just don't work for me well at all. As long as they're accompanied with words like "open" and "close" they're much easier to understand. Otherwise I have to think about them for a couple of seconds, or I press one of them at random, and then the other one if this doesn't work.
I guess if I used the door controls often enough I'd get used to them.
I suspect that many people encountering them for the first time find them hard to understand.
 

zwk500

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What is bugging me now is that the only ground mounted ones I can think of are at Lancaster (P4 and Up Through) and I don't recall having noticed them having the red at the top.
The ones at Lancaster are co-acting with the Gantry mounted ones, so there may be a requirement for the aspects to be in the same sequence for both signals.

EDIT: It would appear there isn't:
1612441401428.png
 

Taunton

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The unusual "L" shape of SN109 is a human factors issue that may have been underestimated in the case of the 1999 collision there.
Design of the Paddington scheme seems to have been done by the interns. Quite why the default lie of the points beyond a red SN109 for Down trains being stopped there was head-on straight into the Up Fast, instead of flank protection, as used for the previous 150 years, into the Down Relief, was never satisfactorily explained at the enquiry.
 

edwin_m

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Many earlier signals were Searchlight, which has of course made a reappearance with current, slightly different technology. These use a single light (so no hood/snow issues) and a mechanically-moving slide with red, yellow and green filters for the aspects. In the days of simple electrics, the connection from the signalbox was done with a single pair of wires for setting the aspect. The colour aspects, inside the signal, were mounted in a U-shaped slide, with red at the bottom, and yellow and green on each side. Positive polarity swung it one way, to the green. Negative polarity swung it the other way, to the yellow. No current and the weighted slide fell to the mid position and displayed red. Bulb power was provided by batteries, and a lot of the ones on open line were approach-lit to save current.
There were also searchlights worked mechanically from the box, the only electricity being to light the lamp. There is one on the preserved GCR at Quorn.
Incidentally the PRM TSI requires that door open and close buttons be fitted in a specific order. Interesting, as they aren't usually colour coded, but I guess if you had some kind of visual impairment which made it difficult to distinguish "<|>" from ">|<" then it would help. In the UK so many units have a derogation from it that it's near useless, however.
I don't think they have to be in a specific order, the issue is the height of the open button from the floor. The earlier Turbostars put them at the bottom to meet this, but the later ones had a smaller button panel which allowed them to be in the more logical place at the top without breaking the rules (at the cost of making them more confusing for people used to the previous arrangement). The symbols on the buttons are tactile so anyone with any visual impairment can in theory still work out which is which.
The only place I can think of with this is Birmingham New St where there also seems to be an approach control on the signals.
The local platforms at Newcastle Central have them too.
 

py_megapixel

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I don't think they have to be in a specific order, the issue is the height of the open button from the floor. The earlier Turbostars put them at the bottom to meet this, but the later ones had a smaller button panel which allowed them to be in the more logical place at the top without breaking the rules (at the cost of making them more confusing for people used to the previous arrangement). The symbols on the buttons are tactile so anyone with any visual impairment can in theory still work out which is which.
No - paragraph 4.2.2.3.1.5 of the Annex states:
If both open and closed door control devices are fitted one above the other, the top device shall always be the open control.
 

edwin_m

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No - paragraph 4.2.2.3.1.5 of the Annex states:
That's a change from the original RVAR standard I mentioned, which the earlier Turbostars had to comply with: https://www.legislation.gov.uk/uksi/1998/2456/regulation/5/made
No control device to enable a passenger to open or close a power-operated door shall be fitted to a regulated rail vehicle unless:

(a)the centre of the control device is not less than 700 millimetres and not more than 1200 millimetres vertically above an imaginary horizontal line extended from the door sill of the relevant doorway;

(b)the control device is operable by the palm of the hand exerting a pressure not exceeding 15 newtons;

(c)the control device or its immediate surround is illuminated continuously whenever it is operable;

(d)the control device contrasts with the surface on which it is mounted; and

(e)the control device is identifiable by touch.

(2) When power-operated doors are closed by a member of the operator’s staff the illumination of each such control device shall cease not less than 3 seconds before the doors start to close.
 

py_megapixel

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21C101

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were also searchlights worked mechanically from the box, the only electricity being to light the lamp. There is one on the preserved GCR at Quorn
I believe that is theoretically a semaphore signal with its arm removed leaving just the spectacle lenses.

They were used in places where it was always gloomy and dark meaning sighting an arm is difficult to do and there will never be enough daylight to stop the oil lamp being seen through the lens.

Although it sounds pedantic it is a major distinction in signalling design. As a "semaphore" for example a starter signal it can show a green aspect if the advanced starter was red. A Day Colour Light Signal would have to be three aspect and show yellow.

The "day" is not generally used but "Day Colour Light Signals" was originally the correct term. This is because Semaphore signals are also colour light signals, but because the oil light is too weak to be seen during the day. The signal has a semaphore arm attached for use in daytime.
 

edwin_m

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I believe that is theoretically a semaphore signal with its arm removed leaving just the spectacle lenses.

They were used in places where it was always gloomy and dark meaning sighting an arm is difficult to do and there will never be enough daylight to stop the oil lamp being seen through the lens.

Although it sounds pedantic it is a major distinction in signalling design. As a "semaphore" for example a starter signal it can show a green aspect if the advanced starter was red. A Day Colour Light Signal would have to be three aspect and show yellow.

The "day" is not generally used but "Day Colour Light Signals" was originally the correct term. This is because Semaphore signals are also colour light signals, but because the oil light is too weak to be seen during the day. The signal has a semaphore arm attached for use in daytime.
There was a picture somewhere on here recently of an old signal in the Severn Tunnel (I think) which was just that.

I can't find a photo of the one at Quorn but it looks more like an electromechanical searchlight and would probably need similar optics as it's in the open air, so as you say an oil lamp and spectacle wouldn't work. I think it's used because it's between the platform edge and the steps, and there isn't enough room for a semaphore arm.
 

21C101

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There was a picture somewhere on here recently of an old signal in the Severn Tunnel (I think) which was just that.

I can't find a photo of the one at Quorn but it looks more like an electromechanical searchlight and would probably need similar optics as it's in the open air, so as you say an oil lamp and spectacle wouldn't work. I think it's used because it's between the platform edge and the steps, and there isn't enough room for a semaphore arm.
Here are some pictures of the Quorn one.

 

John Webb

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.....I can't find a photo of the one at Quorn but it looks more like an electromechanical searchlight and would probably need similar optics as it's in the open air, so as you say an oil lamp and spectacle wouldn't work. I think it's used because it's between the platform edge and the steps, and there isn't enough room for a semaphore arm.
It's visible in this picture (click on it to go to the larger original on the Geograph site):
Arrival at Quorn Station

© Copyright Ashley Dace and licensed for reuse under this Creative Commons Licence.
 

Class 170101

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The ones at Lancaster are co-acting with the Gantry mounted ones, so there may be a requirement for the aspects to be in the same sequence for both signals.

EDIT: It would appear there isn't:
View attachment 89944

Liverpool Street has some ground mounted Co-acting signals approaching on the Up Electric as the main signal is obstructed by the left hand wall. The red aspect on the ground mounted signal is higher than the yellow aspect.
 

philthetube

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Yellow Green Yellow Red. Top to bottom
This layout is to ensure red aspect is not obscured By snow sitting on ‘ hood’ of aspect below As no hood below bottom aspect to allow snow Build up ! Ground mounted main aspects have red at top but no hood below .

hood is like visor / brow or ‘ mud guard’ shaped fitment above aspect
This is what I was told and I have seen a pigeon sitting on a hood obscuring a yellow., so it makes sense
 

Llama

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One other consideration is that a ground mounted signal can cause other issues such as reflection on the railhead - the signal before Deansgate station on the Up Oxford Rd line used to have a co-acting signal in the six-foot. Due to the left-hand curvature of the line there it was reported (and found to be correct) that a single yellow aspect could look like a double yellow to a driver at further approaches.
If I remember rightly the order of the aspects on the co-acting signal was changed, probably about 15 years ago now. The signal in question was abolished a few years back. Only the co-acting head was changed, not its 'master' signal.
 
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