Odd speed limits

[rodent279]

Ok, a question that has always intrigued me, and may well have been raised here before.

Why was the speed limit in Gasworks Tunnel, from the KX station throat, for many years 8mph? Why 8, not 5 or 10?

Also, I believe it was 2mph in the adjacent sidings. Again, why 2, not 5? How was it enforced?

 

Cheers N

[Gwiwer]

I remember the throat at Kings Cross being littered with "8" signs.  It may have been a precaution with 10mph considered too fast over a tight and complex array of pointwork but 5mph too slow and restrictive in terms of pathways in and out.  There is also a sense among many road vehicle drivers that "a little bit over" the posted limit is OK.  

 

Before monitoring equipment was available there may well have been a temptation among rail crews to go that little bit extra.   8 rather than 10 might well have meant trains travelled at 10 not 15.  In addition to which speedometers were not always quite as precise as they are now and not all locomotives even had one.  

 

Similar comments apply to speed limits of 2mph which is considered to be below walking pace but would allow for a shunter to walk safely alongside when required and without any undue haste.  

 

[Edwin_m]

It's about 400 metres from the stops at Kings Cross to the tunnel portal, so the throat speed restriction affects the train over a similar distance (remembering that it can't accelerate until the whole train is clear of the restriction).  400m at 5mph takes roundly 3min, compared to less than 2min at 8mph.  So getting to 8mph allows every move in or out to be nearly 1min quicker ("nearly" because the train would still have to accelerate to and brake from that speed).  Not only does this shorten the train's journey time, it also means part of the station throat is unavailable to other movements for a shorter time.   So if the switches and crossings could be engineered for 8mph but not 10mph (including relevant safety margins) then that would be well worth doing.  This benefit increases with faster-accelerating diesel and electric trains, and is far more significant than introducing intermediate limits at higher speeds such as 38mph or 58mph.  

 

I'm not aware of any other 8mph limits on Network Rail, and these days there is probably a safety standard that forbids them.  Manchester Metrolink uses them, no doubt for similar reasons, but other tramways run in km/h which gives more flexibility in choosing a speed without such non-standard values.  

[Andy Siddall]

The story that I was told and have since heard repeated was that as the throat was quite tight, sighting for speed limit posts in both directions was st a premium.

The beauty of 8 was that it could be read in either direction rather than the numbers 5 and 10 which are obviously directional.

 

Andy

Edited February 10, 2020 by Andy Siddall
punctuation.

[iands]

1 hour ago, rodent279 said:

Ok, a question that has always intrigued me, and may well have been raised here before.

Why was the speed limit in Gasworks Tunnel, from the KX station throat, for many years 8mph? Why 8, not 5 or 10?

Also, I believe it was 2mph in the adjacent sidings. Again, why 2, not 5? How was it enforced?

 

Cheers N

Originally steam locos didn't have speedometers fitted (until much later in life, and even then not all locos were so fitted), and it was the drivers responsibility to "judge" the correct speed - by whatever means possible (i.e., timing the spacing of telegraph poles, lengths of jointed rail, mileposts etc.). However, these might not be available or difficult to judge for certain "slow speed" movements. My understanding is that the driver "related" slower speeds to something tangible, e.g. a trotting horse moves at more or less 8 mph. Given that up to the early '60s (and probably a bit beyond), there were still quite a few horse-drawn vehicles in use up and down the country, a driver could feasibly judge the speed of his engine/train by relating to the speed of a trotting horse. 

 

There may of course be a more scientific or documented reason why "8mph" was chosen (rather than 5mph or 10mph), and I shall watch with interest for other responses as I have often found myself asking the same question, but hadn't asked others for their opinions.

[The Johnster]

8 is really infinity that got drunk and fell over...

[RLBH]

There's a line of thought in some quarters that weird speed limits are better observed because drivers think something like 'that's odd, there must be a good reason for it to be like that' and watch their speed a bit more.

 

That's one of the reasons why roadworks often have similar limits for works traffic. The other, which doesn't apply to railways, is that it helps clarify that it's a limit for works traffic and not for the road users on the other side of the line of cones.

[Talltim]

It’s one of those oddities of the human mind that speed limits should be in base 10. Both hours and miles are essentially arbitrary units anyway. 
A question for the oldies, pre metrification, why weren’t speed limits in base 12 (or 14 or 16)?

[Edwin_m]

15 minutes ago, Talltim said:

It’s one of those oddities of the human mind that speed limits should be in base 10. Both hours and miles are essentially arbitrary units anyway. 
A question for the oldies, pre metrification, why weren’t speed limits in base 12 (or 14 or 16)?

Because you know the second digit is a zero (or a five on the railway) so there's less information to interpret.  And because speedometers have a convenient marker every 10mph so it's easy to see if the needle is above or below it.  

Edited February 11, 2020 by Edwin_m

[Talltim]

24 minutes ago, Edwin_m said:

Because you know the second digit is a zero (or a five on the railway) so there's less information to interpret.  And because speedometers have a convenient marker every 10mph so it's easy to see if the needle is above or below it.  

Speedometers could have the convenient marker every 12 mph...

[Derekl]

A little off topic, but perhaps interesting as far as lack of fitted speedometers goes: way back in the 1960s, the South African Railways had a little problem in the Johannesburg area involving a suburban EMU said to have been speeding (it is all a long time ago, but I think it may have resulted in a fatality). The driver was prosecuted presumably for the equivalent of manslaughter. He pleaded not guilty on the basis that the speedometer in the EMU was not operating - the evidence was that that was a common problem in those EMUs (in passing, along with other problems, such as failing door interlocks allowing trains to run with open doors). My aunt covered the court case as a journalist. The prosecution produced tapes of the noise of the EMUs at various speeds and played them to the driver, who was able to accurately and consistently estimate the speed on the basis of the sound recordings, much to the amazement of my aunt, which is why I recall the story. I think he ended up being found guilty as as result, though....

 

Ans maybe he was helped by having driven EMUs with working speedometers, unlike those driving steam locos here.

[Reorte]

4 minutes ago, Derekl said:

Ans maybe he was helped by having driven EMUs with working speedometers, unlike those driving steam locos here.

 

Whilst they might not've been able to give an exact number to the speed I can easily imagine that a driver would be able to maintain a consistent speed from day to day (varying from place to place as appropriate) from the noise of the loco, the feel, the rhythm over jointed track and so on. Even in a car you're familiar with it's surprisingly easy to judge speed without constantly looking at the speedometer.