Safespeed spoilsports

Discussion in 'UK and Europe' started by Kit Wolf, Feb 26, 2003.

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  1. Kit Wolf

    Kit Wolf Guest

    Anybody else noticed this...

    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.

    Kit
     
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  2. Just Zis Guy

    Just Zis Guy Guest

    On Tue, 25 Feb 2003 21:33:51 +0000, "Kit Wolf" <[email protected]> wrote:

    >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. http://www.chapmancentral.com (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.
     
  3. dja25

    dja25 Guest

    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
     
  4. dja25

    dja25 Guest

    On Wed, 26 Feb 2003 [email protected] wrote:

    > 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
     
  5. Paul Smith

    Paul Smith Guest

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

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

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

    >[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 http://www.safespeed.org.uk please remove "XYZ" to reply by email speed
    cameras cost lives
     
  6. Just Zis Guy

    Just Zis Guy Guest

    Paul Smith wrote:
    > On Tue, 25 Feb 2003 22:48:00 +0000, "Just zis Guy, you know?" <[email protected]> wrote:
    >
    >> [2] the word *change* is omitted in Smith's hilarious 12mph page.
    >
    > I'll review the wording. It certainly isn't intended to mislead.
    >
    >> [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.

    http://www.highwaycode.gov.uk/09.shtml#103 http://www.highwaycode.gov.uk/09.shtml#104
     
  7. Paul Smith

    Paul Smith Guest

    On Wed, 26 Feb 2003 10:37:49 -0000, "Just zis Guy, you know?" <[email protected]> wrote:

    >Paul Smith wrote:

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

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

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

    >>> [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.

    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 protected]> Organization: Safe Speed To: [email protected]

    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:

    http://www.safespeed.org.uk/12mph.html

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

    http://www.safespeed.org.uk/intro.html
    --
    Paul Smith Scotland, UK http://www.safespeed.org.uk please remove "XYZ" to reply by email speed
    cameras cost lives
     
  8. Simon Proven

    Simon Proven Guest

  9. Paul Smith

    Paul Smith Guest

    On Wed, 26 Feb 2003 14:50:34 +0000, Simon Proven <[email protected]> wrote:

    >Paul Smith wrote:

    >> http://www.safespeed.org.uk/12mph.html

    >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 http://www.safespeed.org.uk please remove "XYZ" to reply by email speed
    cameras cost lives
     
  10. Simon Proven

    Simon Proven Guest

    Paul Smith wrote:
    > On Wed, 26 Feb 2003 14:50:34 +0000, Simon Proven <[email protected]> wrote:
    >
    >
    >>Paul Smith wrote:
    >
    >
    >>>http://www.safespeed.org.uk/12mph.html
    >
    >
    >>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.

    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.
     
  11. Paul Smith wrote:
    > On Wed, 26 Feb 2003 14:50:34 +0000, Simon Proven <[email protected]> wrote:
    >
    >> Paul Smith wrote:
    >
    >>> http://www.safespeed.org.uk/12mph.html
    >
    >> 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
     
  12. Paul Smith

    Paul Smith Guest

    On Wed, 26 Feb 2003 15:08:04 +0000, Simon Proven <[email protected]> wrote:

    >>>>http://www.safespeed.org.uk/12mph.html

    >>>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.

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

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

    >Assumption that lower speed limit -> higher impact speeds.

    No such assumption is made.

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

    The Joksch equation? But of course.

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

    Prove it.
    --
    Paul Smith Scotland, UK http://www.safespeed.org.uk please remove "XYZ" to reply by email speed
    cameras cost lives
     
  13. Paul Smith

    Paul Smith Guest

    On Wed, 26 Feb 2003 19:13:59 -0000, "Michael MacClancy" <[email protected]> wrote:

    >Paul Smith wrote:
    >> On Wed, 26 Feb 2003 14:50:34 +0000, Simon Proven <[email protected]> wrote:
    >>
    >>> Paul Smith wrote:
    >>
    >>>> http://www.safespeed.org.uk/12mph.html
    >>
    >>> 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 http://www.safespeed.org.uk please remove "XYZ" to reply by email speed
    cameras cost lives
     
  14. In article <[email protected]>,
    [email protected] says...

    > 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.

    > 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.

    > 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
     
  15. Paul Smith wrote:
    >
    > 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
     
  16. Paul Smith

    Paul Smith Guest

    On Wed, 26 Feb 2003 15:48:02 +0000, Simon Proven <[email protected]> wrote:

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

    >> The Joksch equation? But of course.

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

    No. It predicts fatality rates for delta V

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

    Average Delta V.

    >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.

    >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 http://www.safespeed.org.uk please remove "XYZ" to reply by email speed
    cameras cost lives
     
  17. Paul Smith

    Paul Smith Guest

    On Wed, 26 Feb 2003 19:52:15 -0000, "Michael MacClancy" <[email protected]> wrote:

    >Paul Smith wrote:
    >>
    >> 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?

    Nothing. It says "we COULD kill just as many" not "we WOULD kill just as many".
    --
    Paul Smith Scotland, UK http://www.safespeed.org.uk please remove "XYZ" to reply by email speed
    cameras cost lives
     
  18. In article <[email protected]>,
    [email protected] says...
    > On Wed, 26 Feb 2003 15:48:02 +0000, Simon Proven <[email protected]> wrote:
    >
    > >>>Extrapolation of an average speed across a non-linear function.
    >
    > >> The Joksch equation? But of course.
    >
    > >The Joksch equation does not predict fatality rates for average speds.
    >
    > No. It predicts fatality rates for delta V
    >
    > >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
     
  19. Tony W

    Tony W Guest

    <[email protected]> wrote in message
    news:p[email protected]...
    > 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
     
  20. Paul Smith wrote:
    > On Wed, 26 Feb 2003 19:52:15 -0000, "Michael MacClancy" <[email protected]> wrote:
    >
    >> Paul Smith wrote:
    >>>
    >>> 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?
    >
    > 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
     
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