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PostPosted: Fri Nov 19, 2004 15:38 
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Mod wrote:
Not really. We do not know the EXACT speed most accidents happen at.


OK, so it was ambiguous - it seemed as if you were suggesting in the first instance that most accident speeds were fairly well known, and then in the second instance that most accident speeds weren't fairly well known. Now I see the point you were trying to make, your earlier comments make more sense. Thanks.

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When someone says "I was travelling at 27 miles per hour", depending on context and the way they say it can seem very suspicious. Such suspicious answers are fairly common.


Thanks again, it's these little details which make all the difference in interpreting your earlier comments!



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I don't see the problem with local authorities using their judgement in deciding on a speed limit. You think drivers should use their judgement as to what is safe, but the people who administer the roads should not do this?


No, but I DO think that the road authorities need to be singing from the same songsheet unless there's a very good reason for a local deviation from national policy. I'm not so much bothered about one county setting the limit on a particular type of road to 50 and the neighbouring county setting the limit on a comparable stretch of road to 40, although I'd like to see an end to this as well. I'm more concerned about the inconsistencies which lead to, for example, one county applying a 40/50 limit on a stretch of dual carriageway which is near-motorway standard (i.e. 3 lanes, grade separated junctions, central barrier and no pedestrian access), when other counties are quite happy to leave their less-well-engineered duals at NSL.


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Exactly. Which is precisely what's happening in reality.


So you are pro what is happening right now then?


Absolutely not!

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You seem against local authorities deciding upon local speed limits, but you want speed limits to vary depending on circumstances?


Yes, provided that the baseline limits are set consistently and we don't simply slap a variable limit system on top of the existing inconsistencies. Otherwise you could end up with a scenario where, even in ideal conditions, the raised limit on one stretch of road could still be lower than the normal limit on a comparable stretch of road in another county.


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So you think this should all be administered by national government, and local government should have no control over the speed limits of their own transport system?


Basically, yes. If it's deemed legal to travel at x mph on a stretch of road in one county, it should also be legal to travel at x mph on a comparable stretch of road everywhere else in the UK. It may be necessary for the national agencies to delegate road categorisation to the local authorities in some cases, although checks should take place to ensure the locals aren't taking the piss by placing a road in a lower limit category just for the sake of having a lower limit.

And if that sounds like a lot of work, then that's because it probably is. But the speed limit system we have at present is in a right mess, with limits creeping downwards on a county by county basis with no clear justification and, so it seems, no co-ordination between neighbouring counties. If we don't have a major overhaul of the entire system, then sooner or later what little consistency we still have in setting limits on a national basis will vanish.


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PostPosted: Fri Nov 19, 2004 16:11 
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You are mentally holding "all other conditions" constant. But all other conditions cannot be constant.


So, in our example, when a driver is driving at 30mph down the road he cannot have the same driver reaction time and attention to the road as the driver who is driving at 40mph? When we are looking at how one variable affects the outcome of something we have to hold other things constant, otherwise we can draw no conclusions.

I am not speaking of a driver who has changed speed recently, I am talking about a driver driving down a familiar road at a speed they are familiar with, at a constant speed of either 30 or 40mph.


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PostPosted: Fri Nov 19, 2004 16:38 
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Mod wrote:
Quote:
You are mentally holding "all other conditions" constant. But all other conditions cannot be constant.


So, in our example, when a driver is driving at 30mph down the road he cannot have the same driver reaction time and attention to the road as the driver who is driving at 40mph?


He might or he might not. However if we have altered the behaviour of an INDIVIDUAL, it is CERTAIN that he's applying different judgements or different criteria, otherwise he would still be travelling an the original speed.

Mod wrote:
When we are looking at how one variable affects the outcome of something we have to hold other things constant, otherwise we can draw no conclusions.


Often that's true, but it may lead to dangerous oversimplification in some cases. This is one of those cases.

Look at it this way: Policy affects minds. Minds affect speeds. If we forget the term in the middle and say that policy affects speeds we risk missing out the vital element - what's going on in the mind.

Mod wrote:
I am not speaking of a driver who has changed speed recently, I am talking about a driver driving down a familiar road at a speed they are familiar with, at a constant speed of either 30 or 40mph.


I think you're getting this now aren't you?

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PostPosted: Fri Nov 19, 2004 16:52 
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So, it is your assertion that a driver travelling at 40mph is going to have faster reaction times than if he was travelling at 30mph in the exact same situation? Maybe so. But if you are driving at 30mph and your reaction time 1.5 seconds and your reaction time is 1.2 seconds when you are moving at 40mph, is this going to drastically affect the fact that the final speed at the collision point is going to be higher for the 40mph situation?


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PostPosted: Fri Nov 19, 2004 17:00 
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Mod wrote:
So, it is your assertion that a driver travelling at 40mph is going to have faster reaction times than if he was travelling at 30mph in the exact same situation? Maybe so. But if you are driving at 30mph and your reaction time 1.5 seconds and your reaction time is 1.2 seconds when you are moving at 40mph, is this going to drastically affect the fact that the final speed at the collision point is going to be higher for the 40mph situation?


Nope. We've fundamentally established I hope that reactions and attention are interwoven with mental priorities and that these priorities are interwoven with speed choice. Indeed speed choice is part of the fundamental process of driving. In your example, it may be on occasion that the 30mph driver is checking his speedo at the instant that the incident takes place and may be more than one second late in his reaction.

But that's not really the point I'm trying to get across. We have to look much wider if we are to guess at the overall effects. Remember I mentioned that the average driver goes 150 years between causing injury crashes? What happens to crashes if he becomes more concerned with numerical speed? Will he be as effective at slowing down when necessary? Will he avoid countless millions of crash opportunities as effectively as he did before we messed with his mind?

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PostPosted: Fri Nov 19, 2004 17:55 
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And the guy doing 40 might be looking at his speedo.

I am not attempting to dispute that accidents are relatively rare, not am I attempting to dispute that injuries as a result of accidents are quite low. What I am saying is that speed is a factor in determining the seriousness of the accident. Thus: the faster you are driving, the harder you hit.

I am not saying that speed is the only factor, nor am I saying that it is necessarily the most important factor, but if you are travelling at 40mph you have less margin for error than if you are travelling at 30mph. I appreciate that a driver that spends a large percentage of the time looking at his speedo is not spending enough time assesing risks on the road. But a decent driver should be able to drive at 25mph and keep within 5 mph of that speed most of the time without spending any more time looking at their speedo than someone travelling along with less interest in keeping within the speed limits.


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PostPosted: Fri Nov 19, 2004 18:11 
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Mod wrote:
And the guy doing 40 might be looking at his speedo.

I am not attempting to dispute that accidents are relatively rare, not am I attempting to dispute that injuries as a result of accidents are quite low. What I am saying is that speed is a factor in determining the seriousness of the accident. Thus: the faster you are driving, the harder you hit.

I am not saying that speed is the only factor, nor am I saying that it is necessarily the most important factor, but if you are travelling at 40mph you have less margin for error than if you are travelling at 30mph. I appreciate that a driver that spends a large percentage of the time looking at his speedo is not spending enough time assesing risks on the road. But a decent driver should be able to drive at 25mph and keep within 5 mph of that speed most of the time without spending any more time looking at their speedo than someone travelling along with less interest in keeping within the speed limits.


Arrrgh! You're driving me nuts! :)

Fundamentally, you keep mentally taking the driver out of the loop, and you can't do that - there's always a driver in the loop. He sets his speed according to conditions and instructions and does his best to react to hazards and avoid collisions. He's remarkably effective at doing so. He encounters dozens of hazards each day, yet goes many many years between serious failures. He routinely delivers to himself time to react by observing ahead and adjusting his speed on the basis of the observation.

Maybe one or more of these references will help:

http://www.safespeed.org.uk/why.html
http://www.safespeed.org.uk/inattention.html
http://www.safespeed.org.uk/tiger.html
http://www.safespeed.org.uk/timetoreact.html
http://www.safespeed.org.uk/ten.html

Can anyone else see what's going wrong here and help bridge the gap between Mod's position and mine?

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PostPosted: Fri Nov 19, 2004 18:36 
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I pretty much agree with ten seconds
I also agree mostly with why drivers speed. In that speeding doesn't cause accidents, but speeding can turn a near miss into a collision, a collision into a minor injury case, a minor injury into serious, and serious to fatal.
I agree with Inattention. Not paying attention is a big cause of accidents. When in charge of a machine like a car. Pay attention.
I agree with the tiger document, in that if a driver has to start worrying about new things it could increase accidents, and I agree that speed cameras might cause this to happen in drivers who have previously trained themselves that 35mph is the normal speed for an open residential area road.
As with Inattention, I agree with Time to React, in that reactions times affect how a situation might pan out.


I still believe that the average speed you are travelling, has an affect on the severity on your once in every 7 year accident. I don't believe I am taking the driver out of the loop at all. I am not talking about how speeding causes accidents. I am talking about any driver finding himself in a situation (whatever the reason) where an collision with a hazard is unavoidable. Assuming that in both cases both drivers realize that a hazard is occuring at the same distance away. for example, 15metres away.

I understand that drivers set speeds for themselves effectively every day and are not involved in accidents that often. Speeding doesn't mean that you are automatically going to be presented with a hazard that will involve emergency braking.


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PostPosted: Fri Nov 19, 2004 19:37 
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Mod wrote:
I still believe that the average speed you are travelling, has an affect on the severity on your once in every 7 year accident. I don't believe I am taking the driver out of the loop at all. I am not talking about how speeding causes accidents. I am talking about any driver finding himself in a situation (whatever the reason) where an collision with a hazard is unavoidable. Assuming that in both cases both drivers realize that a hazard is occuring at the same distance away. for example, 15metres away.


I don't diasgree that free travelling speed has "an effect" on impact speed. On average it's a pretty tiny contribution compared with other factors.

What percentage of accidents do you think take place without prior braking?

What percentage of accidents do you think take place without adjusting speed for "area of danger" reasons?

[Let me expand on those two - If I'm approaching a junction with vehicles waiting to pull out, I would typically recognise the potential risk and slow down. If one of them did pull out I would brake. Both behaviours are commonplace and important.]

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PostPosted: Sat Nov 20, 2004 02:55 
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Mod,

I agree with what you are saying provided that all other criteria remain unchanged. We have a "wipe off 5 (3mph) and stay alive" campaign going on over here with advertising showing that at 35mph you wipe off half your speed in the last 5 metres and to demonstrate they have two cars - one at 35mph and one at 38mph. They both brake at exactly the same time in exactly the same type car with a truck parked across the road at about the stopping distance required from 35mph. The car travelling at 38mph hits the rear corner of the truck and it squashes the front of the car back to the windscreen whereas the other vihicle just nudges the truck.

It all seems so logical, but it's misleading in the extreme. If the message is really taken to its limits we should all stop driving because every 3mph of speed increases the risk of a more serious crash.

There is no such thing as an exact stopping distance required for a specific speed. There are far too many variables to take into account. The type of car, the type, condition and temperature of it's brakes, the type, condition and temperature of the tyres, the condition of the suspension, the type of road surface, the condition of the road (potholes, cracks etc), whether the road is wet or dry, whether the road has some sort of spill (eg diesel), the reaction time of the driver, whether the obstacle is stationary or moving, and if so, in which direction etc.

The one thing that I keep coming back to is that the faster I drive the further ahead I look and the more I concentrate. 30 -v- 33mph would be unlikely to make much difference to my reaction time but 40mph would because I would be taking into account hazards much further ahead.

At 30mph I am more interested in pedestrians & vehicles close to me (say 5-70 metres depending on the frequency of parked cars and the age groups of the pedestrians). As the speed increases the majority of pedestrians within 10-15 metres no longer count as it is unlikely that they would get onto the road before I had passed them and I would be spending more time looking at pedestrians and vehicles further ahead (say 15-100 metres once again depending of the frequency of parked cars and the age groups of the pedestrians). Therefore my reaction time to an obstacle in my path would be far less as I would see them further off.

In all of this I have assumed that I was travelling at a speed I felt appropriate and I was not worried about cameras. Now, let’s bring in the cameras and my concern about not exceeding the speed limit by 2mph (some book you for even that little amount) and I am likely to be spending a significant amount of additional time scanning the speedo therefore potentially increasing my reaction time and therefore the likelihood of a crash.

I know this post is long but so is the list of things to be considered every time you are behind the wheel of a moving vehicle. To take any individual component of that list (such as speed) and attempt to draw specific conclusions without regard to the myriad of other issues is a total waste of time and is more likely to be counter-productive than beneficial.

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PostPosted: Mon Nov 22, 2004 12:12 
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OK, you'll be glad to know I've gone away this weekend and thought an awful lot about what has been said.

Instead of replying to all the minutiae of previous commentary let me first attempt to put into my own words what I think Paul is trying to say:

Most people are not involved in serious road accidents, this implies that most people drive in a safe manner.

Driving safely includes selecting an appropriate speed for the road conditions, this implies that most people are capable are selecting said speed.

Therefore, the speed that most people do not exceed (the magic 85th percentile) for any given stretch of road is the speed at which it is probably safe to be driving below, rather than an arbitrary number decided in a committee.

Would you agree with this? I am sure that there is plenty more that you have to say, but is there any part of what I have just said that you do not agree with? Naturally this question applies to everyone involved in this discussion.


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PostPosted: Mon Nov 22, 2004 13:58 
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Mod wrote:
Therefore, the speed that most people do not exceed (the magic 85th percentile) for any given stretch of road is the speed at which it is probably safe to be driving below, rather than an arbitrary number decided in a committee.


I accept the possibility that there are some misleading road environments where the 85th percentile speed is greater than the ideal speed, so I'd have to say:

Therefore, the speed that most people do not exceed (the magic 85th percentile) for almost any given stretch of road is the speed at which it is [delete: probably] safe to be driving at or below, rather than an arbitrary number decided in a committee.

Also note that this behaviour isn't always the same for a given road. In bad conditions, the safe speed and the 85th percentile speeds will be lower.

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PostPosted: Mon Nov 22, 2004 18:32 
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OK, the first premise, that most people are not involved in serious road accidents I agree with. In fact, if I may, I'd like to change it to "Most people are not involved in serious road accidents most of the time". That is meant to accurately reflect your earlier statement that "Most drivers, most of the time are driving not crashing. ".

Most people are not involved in serious road accidents most of the time, this implies that most people drive in a safe manner.

The inference that this means that most people drive in a safe manner is not strictly speaking true, or has yet to be proven. My counterargument runs along the lines of:

A hotel. A hotel I used to work at. This hotel had a habit of blocking fire exits with beds/vacuum cleaners. It had a habit of propping open those fire doors which are designed to compartmentalise the building to minimize the spread of fire. It had a habit of emptying ashtrays into normal bin at the same time as filling the bin with newspapers before leaving the room this happened in unsupervised.

They did this everyday. This hotel has not had many people die in fires, although there have been one or two fires in the past couple of years, they have been controlled and the evacuation has gone almost smoothly...none of those fires were serious fires (they were discovered whilst still small and were in easily controlled areas (one in a kitchen, the other in a toilet)).

So, this hotel is not involved in any serious fires on a daily basis. Does the fact that no fires have occured in its history that have caused death or serious injury mean that its fire attitude is safe? Should we allow hotels to set their own health and safety standards based on what they deem to be adequate? It is unlikely that this hotel is going to suffer a fire that will strip it bare and cause it to crumble into dust (such fires are probably rarer than serious car accidents), but because they are unlikely to be involved in such a situation, does that give them carte blanche to avoid taking adequate safety precautions when they deem them unnecessary?


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PostPosted: Tue Nov 23, 2004 04:55 
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Mod wrote:
My counterargument runs along the lines of:

A hotel. A hotel I used to work at. This hotel had a habit of blocking fire exits with beds/vacuum cleaners. It had a habit of propping open those fire doors which are designed to compartmentalise the building to minimize the spread of fire. It had a habit of emptying ashtrays into normal bin at the same time as filling the bin with newspapers before leaving the room this happened in unsupervised.


This argument is inapplicable. The behaviour that the authorities deem to be dangerous in its own right is speeding. Your example could, I suppose apply to seat belt wearing, where the benefits only become apparent after a crash (or in the hotel case, a fire).

So it's like this:

Blocking a fire exit is dangerous when there's a fire.
Not wearing a seat belt is dangerous if there's a crash.

Contrast with the claim: "Exceeding a speed limit is dangerous."

Latest official figures show that 59% of vehicles were exceeding the 30mph speed limit for example. That's like saying 59% of hotels were on fire...

(Vehicle speed data (lots of it!) linked from this PR:
http://www.safespeed.org.uk/pr123.html )

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PostPosted: Tue Nov 23, 2004 07:43 
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And that's where we have differed then. I am not saying here that speeding leads directly to accidents. What I am saying is that when an accident happens, the speed that you were going plays a huge role in deciding the outcome.


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PostPosted: Tue Nov 23, 2004 07:51 
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Mod wrote:
And that's where we have differed then. I am not saying here that speeding leads directly to accidents. What I am saying is that when an accident happens, the speed that you were going plays a huge role in deciding the outcome.


I see. We've been here before haven't we? All around the houses and back to the start. :)

I don't know if this is different from anything we've been over before, but it's from an email I sent yesterday on a very closely related subject:

"You're assuming that there's no alteration to driver reactions, attention,
concentration, quality or behaviour at the imagined lower speed. Such
assumptions are nowhere near safe.

In fact, I'd go much further and say, there's absolutely no chance that driver
attention, driver concentration and driver quality will be unaffected by
schemes intended to reduce speed.

On this basis the "physics" observation is worse than worthless because it's
misleading.

We assume that things like speed cameras alter vehicles speeds, but that's not
reality either. What they really do is alter drivers' minds, and drivers may
then select lower speeds. All speeds are driver selected, and influencing
drivers' choice of speed actually affects the criteria they use for choosing
speed. It all scares the hell out of me, because we're interfering with road
safety fundamentals (drivers' choice of appropriate speed) with no scientific
knowledge of the wider consequences whatsoever.

Modern road safety thinking is shot through with this sort of oversimplified
blundering and we wonder why road deaths are on the rise!
"

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PostPosted: Tue Nov 23, 2004 08:44 
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I am not discussing the merits or risks of speed cameras here, that can be left until we get the nitty gritty sorted out...

You have stated before that driver attention differs at higher speed, based on the fact that he is doing something different so he has different motivations, so he must be thinking in a different manner. Makes sense. Can you demonstrate that a driver who is driving at or below the speed limit is generally less attentive than a driver who is driving above it?


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PostPosted: Tue Nov 23, 2004 13:21 
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Mod wrote:
You have stated before that driver attention differs at higher speed, based on the fact that he is doing something different so he has different motivations, so he must be thinking in a different manner. Makes sense. Can you demonstrate that a driver who is driving at or below the speed limit is generally less attentive than a driver who is driving above it?


Absolutely not. The key factors have nothing whatsoever to do with the speed limit. If they did, then reducing the speed limit while traffic speeds remained the same would have a dramatic effect on safety. Clearly that would be nonsense.

I say we need appropriate speed choice irrespective of the speed limit. Sometimes lower. Sometimes higher. Always aiming for the optimal. Sometimes enforcing the speed limit will take our drivers out of their optimal ranges.

There are then a complex series of mechanisms, but perhaps this is one of the clearest and one of the most important:
http://www.safespeed.org.uk/forum/viewtopic.php?t=1033

We also have to look at behaviour. Here's a very worrying example of a responsive behaviour to a means of speed reduction:
http://www.safespeed.org.uk/speedo.html

We also have to look at skills acquisition:
http://www.safespeed.org.uk/problem2.html

There's more if you need more.

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PostPosted: Tue Nov 23, 2004 15:11 
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I'll give those a read after work. However I wasn't meaning that the speed limit itself changes someone's attentiveness, but if it is 'safe' (under the 85th percentile definition) to drive at 35 or below, someone who is abiding by this 'speed limit' is more attentive and has better reactions to emergencies than someone driving at or below 30mph, which is the posted, legal speed limit?


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PostPosted: Tue Nov 23, 2004 16:26 
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Mod wrote:
I'll give those a read after work. However I wasn't meaning that the speed limit itself changes someone's attentiveness, but if it is 'safe' (under the 85th percentile definition) to drive at 35 or below, someone who is abiding by this 'speed limit' is more attentive and has better reactions to emergencies than someone driving at or below 30mph, which is the posted, legal speed limit?


Two separate ideas / facts you've muddled together there.

The 85th percentile driver is not safer than average because of his speed. Rather he's an 85th percentile driver because of his attitude, competence and confidence. This applies across the range and at the 5th percentile we find timid underskilled and underconfident drivers for example.

It's the skill set that provides the degree of safety and also infers the place in the speed distribution.

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