Safespeed spoilsports

Kit Wolf said:

Whenever I see a speed camera sign, I think of the safespeed website, see red, and cycle past
calmly at a reasonable speed. I used to enjoy trying to tease the gatsos.

LOL! Here's an interesting site, though, linked from s*f*sp**d -
<http://www.tfhrc.gov/safety/speed/speed.htm>

The author analyses a lot of data from various sources and concludes that changing urban speed
limits makes little difference, increasing highway speed limits increases accidents, and the safest
speed is the median speed or slightly below, which, on a 30mph road, would mean travelling at -
guess what? somewhere below 31mph[1].

I noticed in particular figure 8, showing (amazingly) a rise in the accident rate with increasing
speed, and a drop with decreasing speed.

Also some tables showing, generally, speed limit reductions leading to reductions in crashes, and
speed limit increases leading to rises in crashes.

Also tables showing quite strongly that traffic calming significantly reduces crash, injury and
fatality rates, but that some cagers compensate by driving more dangerously on adjacent streets.

The summary is quite measured:

"There is evidence that crash risk is lowest near the average speed of traffic and increases for
vehicles traveling much faster or slower than average[1]. The occurrence of a large number of
crashes involving turning maneuver partly explains the increased risk for motorists traveling
slower than average and confirms the importance of safety programs involving turn lanes, access
control, grade separation, and other measures to reduce conflicts resulting from large differences
in travel speeds.

"When the consequences of crashes are taken into account, the risk of being involved in an injury
crash is lowest for vehicles that travel near the median speed or slower and increases exponentially
for motorists traveling much faster. One of the major concerns in all of the studies is the travel
speed before the crash. Emerging technology used in mayday, vehicle tracking, and adaptive speed
control systems provide the opportunity to accurately and continuously capture travel speed. This
technology should be applied in improving our understanding of the relationship between speed, speed
variation, and safety.

"When a crash occurs, its severity depends on the change in speed of the vehicle at impact. The
fatality risk increases with the change in speed to the fourth power[2]. International research
indicates the change in injury crashes will be twice the percentage change in speed squared, and
fatal crashes will be four times the percentage change in speed. These relationships are based
mainly on speed limit and speed changes on high-speed roads. More research is needed to assess their
applicability to low-speed urban roads[3].

"In general, changing speed limits on low and moderate speed roads appears to have little or no
effect on speed and thus little or no effect on crashes. This suggests that drivers travel at speeds
they feel are reasonable and safe for the road and traffic regardless of the posted limit. However,
on freeways and other high-speed roads, speed limit increases generally lead to higher speeds and
crashes. The change in speed is roughly one-fourth the change in speed limit. Results from
international studies suggest that for every 1 mi/h change in speed, injury accidents will change by
5 percent (3 percent for every 1km/h). However there is limited evidence that suggests the net
effect of speed limits may be positive on a system wide basis. More research is needed to evaluate
the net safety effect of speed limit changes.

"Most of the speed related crashes involve speed too fast for conditions. This would suggest that
variable speed limits that adjust with traffic and environmental conditions could provide potential
benefits[4].

"Despite the large number of references concerning traffic calming, very few reports include results
of a systematic evaluation. In many cases traffic volumes as well as speed are reduced. As a result
of the traffic diversion, crashes may be migrating to other roads. More research is needed to assess
the system wide impacts and permit comparisons to be made among individual as well as combinations
of traffic calming measures. "

[1] average speed on a 30mph road is 30.9mph, average on a 40mph road is 35.8mph.
<http://www.transtat.dft.gov.uk/tables/tsgb02/4/download/41302.xls>
[2] the word *change* is omitted in Smith's hilarious 12mph page.
[3] i.e. the entire 12mph page is complete bollocks :-D
[4] I said that! see
<http://www.chapmancentral.com/Web/public.nsf/Documents/Safer_Roads_Manifesto> Guy
===
** WARNING ** This posting may contain traces of irony. chapmancentral.comchapmancentral.comOpen ↗ (BT ADSL and
dynamic DNS permitting)
NOTE: BT Openworld have now blocked port 25 (without notice), so old mail addresses may no longer
work. Apologies.

I haven't spent any time looking at the detail, but I suspect Paul Smith is assuming that most
collisions will be "rear-endings". I suspect these are unlikely except on "high-speed" roads.

Assume that the equation is valid under all conditions.

If two cars of equal mass travelling at 30 mph in an opposite direction hit each other head-on, they
will either come to rest or bounce off each other back the way they came. Each car will undergo
delta V of at least 30 mph. If they are going at 40 mph, the delta V will be at least 40 mph.
Assuming the coefficient of restitution is the same at the two speeds, the chance of each driver
dying in the collision is doubled from about 5% to about 10%, according to SafeSpeed's graph.

If the probability of a head-on collision on any given road were the same at each speed, driving at
40 mph would twice as likely to result in fatality.

Something similar will apply to sideways collisions. Increased absolute speed will again result in
increased delta V. I would guess that this is the most likely form of collision?

I suspect the effects will be worse for "vulnerable road users", since they don't have the benefit
of a steel box round them.

Allowing for the increased stopping distance, driving a extra few mph above the speed limit in a 30
mph zone could easily result in a significantly higher-than-normal impact speed. At those speeds it
may not be dangerous for the driver but it could easily be so for a pedestrian.

Paul Smith's logic may work to some extent for high speed roads with a central reservation and few
hazards, but it won't work about town.

DJA

26 Feb 2003 said:

If the probability of a head-on collision on any given road were the same at each speed, driving
at 40 mph would twice as likely to result in fatality.

Having re-read that, I see that it is not quite true since drivers will have a chance to slow down.
However, this is less likely to be the case in sideways collisions.

DJA

you know? said:

[2] the word *change* is omitted in Smith's hilarious 12mph page.

I'll review the wording. It certainly isn't intended to mislead.

Quoted message said:

[3] i.e. the entire 12mph page is complete bollocks :-D

I've just recently traced Hans Joksch to Michigan University. I've emailed him the 12mph url and
invited his comments.
--
Paul Smith Scotland, UK safespeed.org.uksafespeed.org.ukOpen ↗ please remove "XYZ" to reply by email speed
cameras cost lives

Paul Smith said:

you know? said:

[2] the word *change* is omitted in Smith's hilarious 12mph page.

I'll review the wording. It certainly isn't intended to mislead.

Quoted message said:

[3] i.e. the entire 12mph page is complete bollocks :-D

I've just recently traced Hans Joksch to Michigan University. I've emailed him the 12mph url and
invited his comments.

Be sure to check that he agrees his formula can be used at speeds below 20mph (where it is zero
within the limits of experimental accuracy) and that he is content to allow the implicit asseriton
that a blanket 12mph limit would result in drivers no longer using the brakes.

--
Guy
===
I wonder if you wouldn't mind piecing out our imperfections with your thoughts; and while you're
about it perhaps you could think when we talk of bicycles, that you see them printing their proud
wheels i' the receiving earth; thanks awfully.

highwaycode.gov.uk09.shtmlOpen ↗ highwaycode.gov.uk09.shtmlOpen ↗

you know? said:

Paul Smith wrote:

Quoted message said:

Quoted message said:

On Tue, 25 Feb 2003 22:48:00 +0000, "Just zis Guy, you know?" <[email hidden]> wrote:

Quoted message said:

Quoted message said:

Quoted message said:

[2] the word *change* is omitted in Smith's hilarious 12mph page.

Quoted message said:

Quoted message said:

I'll review the wording. It certainly isn't intended to mislead.

Quoted message said:

Quoted message said:

Quoted message said:

[3] i.e. the entire 12mph page is complete bollocks :-D

Quoted message said:

Quoted message said:

I've just recently traced Hans Joksch to Michigan University. I've emailed him the 12mph url and
invited his comments.

Quoted message said:

Be sure to check that he agrees his formula can be used at speeds below 20mph (where it is zero
within the limits of experimental accuracy) and that he is content to allow the implicit asseriton
that a blanket 12mph limit would result in drivers no longer using the brakes.

I've done nothing to lead him. Here's the email:

Subject: Crash Fatality Risk Date: Sat, 22 Feb 2003 01:19:01 +0000 From: Paul Smith
<[email hidden]> Organization: Safe Speed To: [email hidden]

Hiya,

I'm an unconventional road safety campaigner.

I would love to hear your comments on the following web page which uses your fatality risk rule of
thumb from 1993 in an unusual way.

Please see web page:

safespeed.org.uk12mph.htmlOpen ↗

To understand the site and the campaign in more detail, please see:

safespeed.org.ukintro.htmlOpen ↗
--
Paul Smith Scotland, UK safespeed.org.uksafespeed.org.ukOpen ↗ please remove "XYZ" to reply by email speed
cameras cost lives

Paul Smith said:

safespeed.org.uk12mph.htmlOpen ↗

Do you actually believe this nonsense?

Simon Proven said:

Paul Smith wrote:

Quoted message said:

Quoted message said:

safespeed.org.uk12mph.htmlOpen ↗

Quoted message said:

Do you actually believe this nonsense?

As an illustration of the relative unimportance of speed in real crashes? Absolutely.

If you don't believe it you might like to find the flaw in the arguments or the calculations. I'll
amend or remove the page if you find any error.
--
Paul Smith Scotland, UK safespeed.org.uksafespeed.org.ukOpen ↗ please remove "XYZ" to reply by email speed
cameras cost lives

Paul Smith said:

Simon Proven said:

Paul Smith wrote:

Quoted message said:

Quoted message said:

safespeed.org.uk12mph.htmlOpen ↗

Quoted message said:

Do you actually believe this nonsense?

As an illustration of the relative unimportance of speed in real crashes? Absolutely.

Quoted message said:

If you don't believe it you might like to find the flaw in the arguments or the calculations. I'll
amend or remove the page if you find any error.

No consideration of pedestrian, cyclist, passenger or motorcyclist fatality rate.

Assumption that lower speed limit -> higher impact speeds.

Extrapolation of an average speed across a non-linear function.

You really must be joking if you expect anyone to take this stuff seriously.

Paul Smith said:

Simon Proven said:

Paul Smith wrote:

Quoted message said:

Quoted message said:

safespeed.org.uk12mph.htmlOpen ↗

Quoted message said:

Do you actually believe this nonsense?

As an illustration of the relative unimportance of speed in real crashes? Absolutely.

You write: Therefore we could reduce all UK speed limits to 12 mph, enforce them perfectly, and
STILL have exactly the same number of fatal accidents.

This just doesn't make sense. If the speed limit was 12mph and enforced perfectly, the average speed
would be significantly less than 12mph, meaning that the average delta V would be even more
significantly less than 12 mph, meaning that many fewer people would be killed than now.

Unless of course you wish to legislate that speed limits become targets and penalise people for
driving below the speed limit.
--
Michael MacClancy

Simon Proven said:

Quoted message said:

Quoted message said:

>safespeed.org.uk12mph.htmlOpen ↗

Quoted message said:

Quoted message said:

Quoted message said:

Do you actually believe this nonsense?

Quoted message said:

Quoted message said:

As an illustration of the relative unimportance of speed in real crashes? Absolutely.

Quoted message said:

Quoted message said:

If you don't believe it you might like to find the flaw in the arguments or the calculations.
I'll amend or remove the page if you find any error.

Quoted message said:

No consideration of pedestrian, cyclist, passenger or motorcyclist fatality rate.

No need. It's an illustration of what happens to car drivers.

Quoted message said:

Assumption that lower speed limit -> higher impact speeds.

No such assumption is made.

Quoted message said:

Extrapolation of an average speed across a non-linear function.

The Joksch equation? But of course.

Quoted message said:

You really must be joking if you expect anyone to take this stuff seriously.

Prove it.
--
Paul Smith Scotland, UK safespeed.org.uksafespeed.org.ukOpen ↗ please remove "XYZ" to reply by email speed
cameras cost lives

Michael MacClancy said:

Paul Smith said:

Simon Proven said:

Paul Smith wrote:

Quoted message said:

> safespeed.org.uk12mph.htmlOpen ↗

Quoted message said:

Do you actually believe this nonsense?

As an illustration of the relative unimportance of speed in real crashes? Absolutely.

You write: Therefore we could reduce all UK speed limits to 12 mph, enforce them perfectly, and
STILL have exactly the same number of fatal accidents.

This just doesn't make sense. If the speed limit was 12mph and enforced perfectly, the average
speed would be significantly less than 12mph, meaning that the average delta V would be even more
significantly less than 12 mph, meaning that many fewer people would be killed than now.

Unless of course you wish to legislate that speed limits become targets and penalise people for
driving below the speed limit.

At 12mph max, I don't suppose there would be many reasons for driving slower...
--
Paul Smith Scotland, UK safespeed.org.uksafespeed.org.ukOpen ↗ please remove "XYZ" to reply by email speed
cameras cost lives

In article <[email hidden]>,
[email hidden] says...

Quoted message said:

Absolutely. But the assertion is: "With 12 mph speed limits, we could still kill just as many."

Only if every single crass was from 12mph to zero. That clearly is a misuse of the relationship
since the relationship is applicable where delta V is less than V, the relationship stems from that
very fact.

Quoted message said:

The point that crash energy equivalent to 12 mph crashes is all that's needed to kill the number
we DO kill, speaks volumes about the importance of speed in fatal accidents.

No, you have misused the relationship.

Quoted message said:

Yes. That's true, but then 12mph is so very far below expectations that even a gross error due
this would make little difference to the fundamental conclusion.

The conclusion is inescapable: you have incorrectly applied the relationship.

Colin

Paul Smith said:

At 12mph max, I don't suppose there would be many reasons for driving slower...

Traffic congestion, slowing down at junctions, going over speed humps, slowing behind cyclists
............

Face it, if the maximum is 12mph, the average is going to be less than 12mph and the delta V much
less than 12 mph - 6 say? What does that do to your conclusions?
--
Michael MacClancy

Simon Proven said:

Quoted message said:

Quoted message said:

Extrapolation of an average speed across a non-linear function.

Quoted message said:

Quoted message said:

The Joksch equation? But of course.

Quoted message said:

The Joksch equation does not predict fatality rates for average speds.

No. It predicts fatality rates for delta V

Quoted message said:

You are using it to predict fatality rates for average speeds.

Average Delta V.

Quoted message said:

The Joksch equation predicts the fatality rate for a given delta V, not a population wide average
delta V or average speed.

Doh, well if 100 vehicles crash with a delta V of 60mph, 50 drivers will be killed.

If 100 drivers crash, and 50 drivers are killed, then the average delta V could have been as
low as 60mph.

Quoted message said:

Can you not see the problem here? Really?

Are you worried that massive delta Vs can only kill once, yet had the crash energy to kill
many times?

That's true, but the conclusion that we could kill just as many at 12mph remains.
--
Paul Smith Scotland, UK safespeed.org.uksafespeed.org.ukOpen ↗ please remove "XYZ" to reply by email speed
cameras cost lives

Michael MacClancy said:

Paul Smith said:

At 12mph max, I don't suppose there would be many reasons for driving slower...

Traffic congestion, slowing down at junctions, going over speed humps, slowing behind cyclists
............

Quoted message said:

Face it, if the maximum is 12mph, the average is going to be less than 12mph and the delta V much
less than 12 mph - 6 say? What does that do to your conclusions?

Nothing. It says "we COULD kill just as many" not "we WOULD kill just as many".
--
Paul Smith Scotland, UK safespeed.org.uksafespeed.org.ukOpen ↗ please remove "XYZ" to reply by email speed
cameras cost lives

In article <[email hidden]>,
[email hidden] says...

Quoted message said:

Simon Proven said:

Quoted message said:

>Extrapolation of an average speed across a non-linear function.

Quoted message said:

Quoted message said:

The Joksch equation? But of course.

Quoted message said:

The Joksch equation does not predict fatality rates for average speds.

No. It predicts fatality rates for delta V

Quoted message said:

You are using it to predict fatality rates for average speeds.

Average Delta V.

No, you are using it for average V that is absolutely clear from that page. If (average) V was 12mph
then (average) delta V would be less.

Colin

<[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

I haven't spent any time looking at the detail, but I suspect Paul Smith is assuming that most
collisions will be "rear-endings". I suspect these are unlikely except on "high-speed" roads.

Assume that the equation is valid under all conditions.

If two cars of equal mass travelling at 30 mph in an opposite direction hit each other head-on,
they will either come to rest or bounce off each other back the way they came. Each car will
undergo delta V of at least 30 mph. If they are going at 40 mph, the delta V will be at least
40 mph. Assuming the coefficient of restitution is the same at the two speeds, the chance of
each driver dying in the collision is doubled from about 5% to about 10%, according to
SafeSpeed's graph.

snip

God god, man -- don't confuse things with analysis.

T

Paul Smith said:

Michael MacClancy said:

Paul Smith said:

At 12mph max, I don't suppose there would be many reasons for driving slower...

Traffic congestion, slowing down at junctions, going over speed humps, slowing behind cyclists
............

Quoted message said:

Face it, if the maximum is 12mph, the average is going to be less than 12mph and the delta V much
less than 12 mph - 6 say? What does that do to your conclusions?

Nothing. It says "we COULD kill just as many" not "we WOULD kill just as many".

The right answer is, "we wouldn't kill as many".
--
Michael MacClancy