Safe Speed Forums

I think I've found some better terminology to explain the relationship between safe speed and urban speed limits.

We've usually said that urban speed limits are required and so set because of "hazard density". It's true and it's sensible, but I've always found it rather difficult to picture hazard density.

The new term is "certainty distance".

It connects better to the safe speed rule: "Always be able to stop comfortably on your own side of the road within the distance that you know to be clear."

"Certainty distance" is about the "knowing" part of the safe speed rule. Where hazard density is high, we can't know the situation so far ahead. That's the real reason for urban speed limits.

Any comments? Can anyone improve the terminology still further?

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Yes, that is a good term. I know exactly what you mean.

But "hazard density" is well-established in advanced driving circles, so should not be disparaged.

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"Show me someone who says that they have never exceeded a speed limit, and I'll show you a liar, or a menace." (Austin Williams - Director, Transport Research Group)

Any views expressed in this post are personal opinions and may not represent the views of Safe Speed

Do you have a mental picture of "hazard density"? I've always found it rather vague and nebulous. It's perfectly possible it's just me.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Yes, I certainly do.

It's hard to define exactly, but it's a combination of level of development, closeness of buildings to the road, number of pedestrians, number of side turnings, number of parked cars etc.

It's a crucial factor in determining which suburban roads are suitable for 40 limits.

It's a phrase widely used within the advanced driving community and to me is very meaningful. But "certainty distance" is perhaps a more precise definition of the same concept.

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"Show me someone who says that they have never exceeded a speed limit, and I'll show you a liar, or a menace." (Austin Williams - Director, Transport Research Group)

Any views expressed in this post are personal opinions and may not represent the views of Safe Speed

Exactly. Nebulous. Do you think it's nebulous?

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

NO! If I'm driving along a road I have a very good idea what the hazard density is. I could probably mark it on a 1-5 scale if you wanted.

It's similar to the "safe speed", really. IMV you can't choose a safe speed (where that at least partly depends on factors other than road alignment) until you've assessed the hazard density.

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"Show me someone who says that they have never exceeded a speed limit, and I'll show you a liar, or a menace." (Austin Williams - Director, Transport Research Group)

Any views expressed in this post are personal opinions and may not represent the views of Safe Speed

This is fascinating, although I suspect we are somewhat at cross purposes.

You said: "hard to define". I said "nebulous". By nebulous, I mean "hard to define, especially around the edges". In that much, I'm pretty sure we agree.

Like you, I know high hazard density when I see it. Nevertheless, trying to write a set of rules to enable an inexperienced driver to recognise high hazard density would be pretty close to impossible. This is why it's hard to define. That's why it's nebulous.

We (you and I) use plenty of sophisticated experience to make our judgements and that's the missing element for new drivers.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

I know a dead parrot when I see one...!

But seriously, here's another term to chuck in the melting pot then: I often think of my "safe space", this being the zone in front of me that I am about to drive into, and that I have mentally swept clear of hazards.

In terms of (nebulousness? nebulosity? what is the adverb?), err, how nebulous this is , then perhaps an interesting exercise is to visualise drawing a mental dotted line around this "safe space" as we drive along, or perhaps imagining it shaded in a different colour. This forces us to make decisions about where the border lies, and immediately we start to see where all the grey areas are. For example, what colour do we shade in the area just in front of that parked car? It's clear now, but we can't know it to be clear as we can't see in front of the car to know what might leap into it. So this technique draws us towards this danger area and we have to exclude this from our "Safe Space". If this then clashes with the required safe space as determined by the speed we are travelling, then we either have to slow down in order to reduce the safe space requirement, or we have to steer further out, to remove this area from our "required" safe space.

Does this help?

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That’s fine but are you sure you want to use 'certainty distance' for this? There is no guarentee that what you (the driver) perceive of as a certainty actually is certain.

When you start a journey, you can be certain that you will need to replan and change many times, checking ahead to see if and how the next leg should be completed, where each leg is the length of the certainty distance ahead. As soon as new facts come to light, the next leg starts, so the journey is an almost infinite number of checks, with adjustments at the start of one. That is the nature of driving, although I don't think of it like that when I am driving. It is tiring if you experience a lot of 'short' certainty distances, so one design goal provides long distances of certainty. This minimises change and reduces planning, stress and effort.

Drivers apply a set of filters at the start of each leg to create a plan that is certain enough to give a very good chance of success. One way to increase certainty distances is to ask drivers to conform to a reliable pattern of driving that other drivers can count on when planning. The quality of the filters, the planning abilities and the objective function (certain enough?) vary greatly from driver to driver, and that is a problem - behavioural consistency - that is partially addressed by speed limits, even though they are not rationalised that way.

The big question on this site is whether to ask drivers to conform to the limits, or to compel them to. Or, rephrased, if drivers are not compelled to obey speed limits, would they be tempted ignore them, as they have in the past?

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No, surely the big question is how much does it matter?

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Yes, closely coupled with: "how do we make it better?"

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

"Always be able to stop comfortably on your own side of the road within the distance that you know to be clear."


Your "safe speed rule" page says "This suggests that we need to add another condition: within the distance that you know to be clear and will remain clear." but the phrase does not actually appear in the re-statement of the rule immediately below. Why not?

Actually, I think the addition should be "... and can reasonably expect will remain clear". This accords with reality and allows for single track roads and other hazards which obstruct visibility.

I don't like the terms "certainty distance". There is often no 'certainty'. And it's a lot harder to guess at the meaning of "certainty distance" than it is to guess the meaning of hazard density and apply the safe speed rule in the context of the hazard density.

If I close my eyes and think of "hazard density" - my mind's eye will throw up a picture of a road where there are parked cars, traffic lights, zebra crossings, a road narrowing, a works entrance, a pub and some shops. I would rate the hazards as well in terms of risk.

If i close my eyes and think "certainty distance" ...my reaction is "can i stop the car in time?"

I have stopped in my tracks and asked my 17 year old son what he understand by these terms - as he is a young and relatively inexperienced driver.

Think his reply is relevant as he falls into the competent but with a lot to learn category:

My son has told me that he understands the term hazard density as meaning the number of hazards he can reasonably expect to see on any road and be required to adjust his driving to deal with!

In response to "certainty distance" - he said he is not sure of the meaning - but would interpret that as being able to stop in the distance he can see to be clear.

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Having thought about this further, the two are quite separate concepts.

"Hazard density" determines the general speed at which it is safe to travel along a road, "certainty distance" is more specific in terms of the response to particular hazards.

I would also echo Observer's point that in many circumstances you can't be absolutely certain that the road space you are entering is going to remain clear, so the qualification "... and can reasonably expect will remain clear" is valid.

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"Show me someone who says that they have never exceeded a speed limit, and I'll show you a liar, or a menace." (Austin Williams - Director, Transport Research Group)

Any views expressed in this post are personal opinions and may not represent the views of Safe Speed

Are you sure you're failing to see the wood for the trees?

Imagine if the only hazards were trees and there were no roads.

The hazards density in an open field would be low, in a wood medium, and in a dense forest high.

The hazard density is simply the density of hazards.

Or am I missing something?

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I've moved this topic to the Brainstorming section because frankly "certainty distance" doesn't seem even half as good as it did late the other evening!

I still have serious reservations about the term "hazard density".

One of the problems my "hind brain" has identified since the topic started is that we're often not talking about REAL hazard density; instead we're talking about the density of POTENTIAL hazards.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Paul, I've always thought by "hazard density" you were talking about potential hazard density. The actual hazard density is normally going to be less, possibly a great deal less. Look at it this way, driving past a school around chucking out time, the risk of kids running into the road increases the potential hazard density and a sensible driver adjusts speed, considers escape areas etc just in case. Most of the time nothing happens and the potential hazard never becomes a real one. I think this comes across well when you say that drivers should set a speed at which they can stop safely in the distance they can reasonably expect to be clear (apologies if I've re-worded that by accident).

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Yes, I think that's one of the things that we've always known, but never stated explicitly.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

hmm.. interesting one...

'certainty distance' ?
often on residential roads i can see length of the road to be clear, no doubt i could stop in that distance at 30mph. however the *ahem* hazard density means i'm more likely to pootle at 20.. what triggers this i guess is not what iu can see to be clear at present, but the amount i am 100% certain will be clear when i get to it. on a parked up residential road this isn't usually much more than a cars length.

'hazard density' ?
i'd kind of assumed this was a standard term when i first read it, and it kind of makes sense to me as a combination of factors:
environmental (weather, road surface)
road type
traffic
and yes, potential hazards
maybe visible distance too.

so residential immediately bumps the score up, poor weather bumps it up, not being able to see very far bumps it up, heavy traffic bumps it up.

if i visualise the above street i'm thinking:
wet road, residential, oncoming traffic in middle of road, not totally clear view of the road, and maybe 100 potential hazards in the distance that i can see (pedestrians, parked cars, houses, gaps between parked cars).

potential hazards would give an interesting measure hazards per metre or something, but that wouldnt take into account that you can only see 10metres! and other more generic 'risks'

just my 2p worth.

That's really why I landed on the term 'certainty'.

However as you'll see above, I think we gained more from the spin offs than the phrase itself.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety