Relationship between speed and safety

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Postby petes » Mon Jun 01, 2015 12:21 pm


As martine loosely suggested, I thought I'd start a new thread on this topic!

The general nature of this post being:

Is there a relationshop between speed and safety?
Are drivers that drive within speed limits safer than those that don't?
Does higher speed increase the risk of accidents?
Is it right that enforcement (through cameras or other means) appears to focus specifically on speed?
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Postby Horse » Mon Jun 01, 2015 12:31 pm


Google Scholar throws up loads of stuff, including:


http://www.sciencedirect.com/science/ar ... 7505001247
Driving speed and the risk of road crashes: A review
Letty Aarts, Ingrid van Schagen

Driving speed is an important factor in road safety. Speed not only affects the severity of a crash, but is also related to the risk of being involved in a crash. This paper discusses the most important empirical studies into speed and crash rate with an emphasis on the more recent studies. The majority of these studies looked at absolute speed, either at individual vehicle level or at road section level. Respectively, they found evidence for an exponential function and a power function between speed and crash rate. Both types of studies found evidence that crash rate increases faster with an increase in speed on minor roads than on major roads. At a more detailed level, lane width, junction density, and traffic flow were found to interact with the speed–crash rate relation. Other studies looked at speed dispersion and found evidence that this is also an important factor in determining crash rate. Larger differences in speed between vehicles are related to a higher crash rate. Without exception, a vehicle that moved (much) faster than other traffic around it, had a higher crash rate. With regard to the rate of a (much) slower moving vehicle, the evidence is inconclusive.



http://www.sciencedirect.com/science/ar ... 759800058X
Speed, speed limits and road traffic accidents under free flow conditions
A.A.M Aljanahia, A.H Rhodes, A.V Metcalfe

The relationship between various measures of traffic speed, under free flow conditions, and accident rate is investigated for two groups of sites, one in the Tyne and Wear county of the UK and the other in Bahrain. The effect of speed limits on traffic speed is estimated for both groups of sites. In Bahrain, there is statistically significant evidence of an association between mean speed and accident rate. In Tyne and Wear the statistical evidence is weaker, and points to a stronger relationship between accidents and the variability of traffic speeds. In both areas, there is an apparent decrease in accident rate if the percentage of heavy vehicles increases, with the speed distribution held constant. In both areas the effect of speed limits is to reduce the mean speed of traffic by at least one quarter. Higher speeds are associated with longer trips.


http://20splentyforus.org.uk/UsefulRepo ... idents.pdf
http://trid.trb.org/view.aspx?id=426230
http://www.bmj.com/content/339/bmj.b4469.full
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Postby akirk » Mon Jun 01, 2015 1:45 pm


it is too simplistic a concept...

- take 100 people - in the same car, same place & conditions & same speed - % risk of an accident will vary
- any one person - 100 different cars etc. - varying risk
- any one person - same car - different conditions
- any one person, same car, same general conditions, but varying stress / tiredness / etc.

too may variable to just say speed is an issue...

and for most accidents it is not directly speed that causes the issue - just the easiest thing to measure / criticise - laziness leads people to say that speed causes accidents...

I was once involved in an accident when a pedestrian ran out between two vans and hit my car... ironically I was doing c. 22mph in a 50mph zone - had I been going faster the accident would not have happened... but I was being cautious!

so based on that I would advoacte higher speeds as being safer - easy to twist statistics

Advanced Driving doesn't focus on speed, it focuses on all the other (many) attributes of keeping safe, and speed slots in neatly and automatically with the others...

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Postby Horse » Mon Jun 01, 2015 1:53 pm


StressedDave wrote:
petes wrote: 3. Does higher speed increase the risk of accidents?


3. No, only the result of said accidents


At higher speeds (for the same circumstances) a driver will have a higher mental workload since the number of hazards won't have reduced.

Also, higher speed results in longer thinking distances and extended braking distances, so even if a hazard is correctly identified, the driver may not be able to react sufficiently early.
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Postby jont » Mon Jun 01, 2015 2:18 pm


Horse wrote:
StressedDave wrote:
petes wrote: 3. Does higher speed increase the risk of accidents?


3. No, only the result of said accidents


At higher speeds (for the same circumstances) a driver will have a higher mental workload since the number of hazards won't have reduced.

Also, higher speed results in longer thinking distances and extended braking distances, so even if a hazard is correctly identified, the driver may not be able to react sufficiently early.

What about the risk of low mental workload turning into disinterest? Thinking motorways in particular where often significantly higher speeds might be safe, but instead cruise control goes on and if not careful, you "switch off". See also queues of lemmings on trunk roads all doing 40/45 mph.
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Postby martine » Mon Jun 01, 2015 5:20 pm


Speed in itself is not dangerous - inappropriate speed most definitely is.

I think the problem is the disengagement of speed limits and safety. We have all come across roads with inappropriate speed limits - which are widely ignored by a significant proportion of drivers. Since the 85% rule has been abandoned many feel the 'number on a stick' bears little relation to how 'hazardous' a particular stretch of road is.

I believe this is a dangerous 'disconnect' - as drivers get used to ignoring speed limits. For example, on rural road there are often 'extended' 30s around villages - it's just possible some of these are there to indicate an additional hazard on the way out - but many are not and are there simply to encourage opposing traffic to get their speed down before the real village starts. :roll:

Also, those sticking religiously to the limit can themselves create danger - especially if the majority are ignoring it.

We need to re-link the driver's mind with hazard perception and speed and not speed limit.
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Postby Horse » Mon Jun 01, 2015 5:59 pm


jont wrote: Thinking motorways in particular where often significantly higher speeds might be safe, but instead cruise control goes on and if not careful, you "switch off". See also queues of lemmings on trunk roads all doing 40/45 mph.


Are either of those known high-risk situations which figure highly in the accident stats?
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Postby jont » Mon Jun 01, 2015 6:02 pm


Horse wrote:
jont wrote: Thinking motorways in particular where often significantly higher speeds might be safe, but instead cruise control goes on and if not careful, you "switch off". See also queues of lemmings on trunk roads all doing 40/45 mph.


Are either of those known high-risk situations which figure highly in the accident stats?


See the major pileups on motorways? I'd imagine if drivers had been paying attention they probably wouldn't have happened. Isn't it just one step on to falling asleep at the wheel?
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Postby TheInsanity1234 » Mon Jun 01, 2015 6:07 pm


I have found, during motorway drives to Essex, when I'm doing 80ish whilst using the accelerator to maintain my speed, I'm much more focused on driving smoothly and varying my speed to minimise potential conflict situations. However if I've set cruise control at 50 or 60 (i.e. managed motorways/roadworks) then I do tend to catch myself fading out a bit, and I have to work hard to focus on the situation around me.

But of course, I don't think it's the actual speed itself which is a factor on motorways, I think it's relative speeds, as in roadworks, I'm most likely going to be travelling at the same speed as the majority of vehicles around me, whereas doing 80 on a quiet-ish motorway means I am moving through the traffic, so constantly looking out for new hazards developing, which in turn leads to me being naturally more attentive.

The effect is the same, for me, even on slow dual carriageways (A127 for instance) as travelling at about 65 mph means you're moving faster than the majority of traffic on that particular road, so the same result is achieved, I find myself naturally focusing more on possible dawdlers who just change lanes regardless of how fast you're approaching, and other hazards too.

Just my £0.02 worth.

Other, more experienced, people may find different things to be true. :)
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Postby daz6215 » Mon Jun 01, 2015 6:34 pm


Take at look at James Reasons Swiss Cheese model on human error.
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Postby JamesAllport » Mon Jun 01, 2015 6:35 pm


Martin was spot on when he said this:

martine wrote:Speed in itself is not dangerous - inappropriate speed most definitely is.

...

We need to re-link the driver's mind with hazard perception and speed and not speed limit.


I'm in the middle of preparing for an IAM National Observer assessment and a Masters assessment so, however I might normally drive, I've been spending time ensuring I'm rigidly within the limits while practicing.

I really dislike the lack of flexibility I find this gives me on motorways in particular. It's not that I particularly want or need to travel faster than 70mph, it's just that having the flexibility to add a few mph (or lose a few) makes not being alongside other traffic for long periods much, much easier.

In West Sussex, we still have relatively sensible speed limiting on our rural roads. But my favourite bit of NSL B road round the corner from my house for training associates on limit points has just been 40 limited, so I now use another fantastic road about 3 mile further on. The irony is this road is single track, and significantly more challenging and technical - but still NSL.

And no, that's not a vote to reduce it. My attitude can probably be deduced from the fact that my 7 year old (who's commentary isn't bad) refers to NSL signs as "Full speed ahead signs". :evil:

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Postby Horse » Mon Jun 01, 2015 7:10 pm


jont wrote:
Horse wrote:
jont wrote: Thinking motorways in particular where often significantly higher speeds might be safe, but instead cruise control goes on and if not careful, you "switch off". See also queues of lemmings on trunk roads all doing 40/45 mph.


Are either of those known high-risk situations which figure highly in the accident stats?


See the major pileups on motorways? I'd imagine if drivers had been paying attention they probably wouldn't have happened. Isn't it just one step on to falling asleep at the wheel?


Aren't mways the safest roads we have?
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Postby chosulman » Mon Jun 01, 2015 10:07 pm


This hasn't been aired on the forum for some time. Lots of newbie MPs were recently elected to power. I wonder if one or two might stake their claim to political immortality or even usefulness by championing the 'Safe Speed' cause?
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Postby fungus » Mon Jun 01, 2015 10:43 pm


I'm with Martin on this one.

Although a speed limit should reflect areas of increased danger, and generally I suppose they do, the fact of the matter is that a SAFE speed will vary from minute to minute due to the fluctuation in hazard density.

There is also the mindset that " I'm keeping to the speed limit so I'm safe." This has been borne out on several occasions by learners who I have taken down a short, 400yds long lane that links two 30mph roads. The only reason that this lane is NSL is the fact that it has no street lamps.
The usual comment is " This road should have a 30 limit " despite the fact that you are unlikely to excede 20 mph because of the narrowness, road surface, and visibility etc.

The important thing is that drivers drive at a speed appropriate to conditions.
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Postby martine » Mon Jun 01, 2015 10:46 pm


JamesAllport wrote:...My attitude can probably be deduced from the fact that my 7 year old (who's commentary isn't bad) refers to NSL signs as "Full speed ahead signs". :evil:

:lol: - at least he didn't say....G...L...F....
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