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PostPosted: Sat Aug 06, 2005 22:11 
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stevei wrote:
SafeSpeed wrote:
If the train in front takes 20 seconds to come to a halt too, you'll be fine.

But I thought you agreed with the principle of only driving at a speed that means you can stop in the distance you can guarantee to be clear? If you have a multiple vehicle pile up, once cars start hitting other cars that have already stopped, they will stop in much less than their normal braking distance. I tend to like to leave a nice big gap to the car in front, getting close to it doesn't really advance my journey, the only thing that makes me get closer than I'd like is that people pull into the gap, so you end up forced to drive at a distance from the car in front that avoids having car after car dive in front of you.


Yep, you have to be able to stop within the distance that you know to be clear - but you must also observe way beyond the vehicle in front.

The two are fully compatible - you need to know that the vehicle in front will clear your safe braking zone before you need to occupy it. It's all perfectly normal actually...

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PostPosted: Sat Aug 06, 2005 22:13 
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PeterE wrote:
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Pete317 wrote:
Taking it a bit further, the braking distance from 40mph is 17.8 metres (assuming zero reaction time) and a 2-second gap at 40mph is 35.6 metres.

By way of a complete aside, I find it rather bizarre to quote a speed in imperial units and the distance in metric units - both in the same sentence. I'm not having a go - many people make that choice, but I personally find it incongruous.

In that context, it's quite understandable, as scientific concepts such as deceleration are usually expressed in metric, but speed limits, and concepts of speed, on UK roads, are Imperial.


Does being understandable stop it being incongruous? It doesn't for me.

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PostPosted: Sat Aug 06, 2005 22:14 
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SafeSpeed wrote:
Well, they were wrong, wrong, wrong then.

The 2 second gap allows time to react. It's on the generous side to allow for unavoidably inattention (mirror checks for example) and for braking performance differential.

It's got absolutely nothing to do with braking distance/time.

Ah, I see the misunderstanding, I should have said stopping distance not braking distance, I knew what I meant while I was typing it :)


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PostPosted: Sat Aug 06, 2005 22:15 
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SafeSpeed wrote:
By way of a complete aside, I find it rather bizarre to quote a speed in imperial units and the distance in metric units - both in the same sentence. I'm not having a go - many people make that choice, but I personally find it incongruous.


Most engineering is done in SI units - which means distances in metres.
Speed should, by the same token, really be in km/h, but we use mph in this country so I use mph so everyone can identify with it.

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PostPosted: Sat Aug 06, 2005 22:17 
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buckmac wrote:
And I don't see how extra rigorous speed enforcement is going to reduce aggressive driving - in fact, I'd rather tend to the idea that it might increase it!

That tallies with what I saw in Anglesey. Locals "in the know" rigidly obeying speed limits, due to draconian speed limit enforcement and cunningly placed Talivans. Tourists not "in the know" get impatient with being stuck in long queues of slow drivers, take risks, and overtake somewhere at best inappropriate and at worst dangerous. Every so often it turns out to a risk too far.

Of course the fatal smashes are seldom anywhere near the camera locations, which are placed to maximise revenue rather than address specific safety issues. (One on the Menai Bridge - Amlwch road is on a dead straight NSL section with no obvious hazards in either direction as far as the eye can see - in other words an ideal "overtaking straight" - indeed one of the few points on that road where it is safe to overtake a queue of vehicles.) And as - they argue - the fatal smashes are clearly solely due to "excessive speed", they are then used to reinforce the flawed argument that excessive speed is the sole cause of accidents, and hence as justification for their enforcement policy.


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PostPosted: Sat Aug 06, 2005 22:25 
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For interest, just derived a table of stopping distance (from highway code) vs 2 sec distance (using full stops instead of spaces because it seems to strip multiple spaces out).

Speed(mph).....Stopping Distance(m).....2 sec distance(m)
......20........................12...............................18
......30........................23...............................27
......40........................36...............................36
......50........................53...............................45
......60........................73...............................54
......70........................96...............................63

As you can see, if cars are separated by stopping distance rather than 2 sec distance, the separation will increase much more with speed, leading to reduced throughput at higher speeds compared to 2 sec separation.

Many modern vehicles block your view of the road ahead, so in these cases a stopping distance gap would be more appropriate than a 2 sec gap.


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PostPosted: Sat Aug 06, 2005 22:44 
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stevei wrote:
Many modern vehicles block your view of the road ahead, so in these cases a stopping distance gap would be more appropriate than a 2 sec gap.


It's rare indeed not to be able to observe past the vehicle ahead.

And don't forget what we need to observe for - the sort of thing we need to know doesn't exist is the sort of thing that can give a stop in less than a braking distance - stationary traffic for example.

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PostPosted: Sat Aug 06, 2005 23:55 
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Pete317 wrote:
Most engineering is done in SI units - which means distances in metres.
Speed should, by the same token, really be in km/h,


If you're strictly using SI units, then speed should be in m/s (metres per second)


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PostPosted: Sat Aug 06, 2005 23:59 
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There is a strong argument for the voluntary use of metric measurements.

There is no argument save that of the jackboot for enforcing their use via the criminal law.

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PostPosted: Sun Aug 07, 2005 00:01 
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stevei wrote:
As you can see, if cars are separated by stopping distance rather than 2 sec distance, the separation will increase much more with speed, leading to reduced throughput at higher speeds compared to 2 sec separation.


But, besides anything else, by and large drivers do not drive according to stopping distance, so - in ther real world - throughput cannot have any relationship to speed.

In fact, there is a tendency for traffic to bunch up at lower speeds on the motorway - which temporarily increases throughput.
But when traffic is bunched up then small perturbations, overreactions etc are much more likely to bring the traffic to a complete stop - at which point the throughput falls to zero.

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Last edited by Pete317 on Sun Aug 07, 2005 00:10, edited 1 time in total.

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PostPosted: Sun Aug 07, 2005 00:09 
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Pete317 wrote:
stevei wrote:
As you can see, if cars are separated by stopping distance rather than 2 sec distance, the separation will increase much more with speed, leading to reduced throughput at higher speeds compared to 2 sec separation.

But, besides anything else, by and large drivers do not drive according to stopping distance, so - in ther real world - throughput cannot have any relationship to speed.

This is something on which meaningful research could be done - to study motorway situations with very high traffic flow and establish the real-world separation times.

I suspect as traffic goes slower drivers do instinctively reduce the separation times, but as motorway traffic rarely flows smoothly below about 40 mph it may be hard to derive firm conclusions.

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PostPosted: Sun Aug 07, 2005 09:55 
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SafeSpeed wrote:
stevei wrote:
Pete317 wrote:
I've often wondered how they come to that conclusion - as there appears to be no logic behind it.
Assuming a 2-second gap.....

I saw it derived on an Open University programme once, as an illustration of how to solve the relevant equations. It was based on people leaving a gap corresponding to their braking distance. The advice nowadays tends to be to leave a much smaller gap than this, e.g. the 2 second rule as you mention, which will result in a different speed to maximise throughput.


Well, they were wrong, wrong, wrong then.

The 2 second gap allows time to react. It's on the generous side to allow for unavoidably inattention (mirror checks for example) and for braking performance differential.

It's got absolutely nothing to do with braking distance/time.


Paul you amaze me :o

What about the speedo check - so often necessary when the scamerati are about!

Cheers, Chris.

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