Safe Speed Forums

Papers published by the Royal Statistical Society include an estimate for Speed Camera RTTM of 18% on KSI collisions from forthcoming research.

See: http://www.rss.org.uk/rssadmin/uploads/ ... 0nov04.pdf Page 21.

This shows RTTM for fatal and serious crashes of -18%, with 95% confidence limits of -25%, -9%. (i.e. the true value lies in the range -9 to -25%).

Net benefit of the camera is calculated as -6% and 95% confidence limits are: -21%, +12%. Yes, that's right folks, it's entirely possible that crashes even at camera sites went up as a result of the camera.

Worryingly, there's also a traffic flow difference of -5%, and this would normally indicate that 1 in 20 vehicles diverted to a less safe road because of the camera.

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Paul Smith
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So if we can presume that a 5% reduction in traffic will give a 5% reduction in accidents, is it reasonable to say that then the net benefit is actually -1% ? Is this a reasonable assumption, or is there a better statistical correlation available between traffic levels and accident incidence, all other things being equal.

Either way, this would seem to indicate that cameras make pretty much no measurable difference whatsoever, as the value is so small (whether positive or negative) as to be all but swamped by RTTM.

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No - I suspect that would be subtracting the benefit due to 'change in flow' twice. This particular table demonstrates only the reduction in total accidents. It does not indicate in any way the benefit due to reduced severity (due to lower average speeds) of those accidents that do occur.

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Actually there's a stong indication of an increase in crash severity. The claim is that injury crashes are significantly reduced while 'ksi' crashes are reduced insignificantly. That's clearly an increase in average crash severity.

When are these people going to wake up?

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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

Here's a view based on Linda mountain's findings:

DfT say crashes down at camera sites by 40% leading to 100 lives per year saved by 5,000 cameras.

Linda says, no, it's only 6%.

6/40 * 100 = 15 lives saved.

5000/15 = 1 life saved per 333 camera years.

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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

Very interesting analysis which can be quoted as only a 6% reduction in FSA (fatal & Serious?)

We know that some camera sites have also had road improvements.

I did not notice that mentioned in the analysis.

The link between average speeds and accident severity exists only in your imagination.
It would only hold true in a world where everybody drove around with eyes glued to their speedos and not paying the slightest attention to the road ahead. And the number of casualties would probably be two or three orders of magnitude higher than they are.
But as most drivers, most of the time, watch the road and adjust their driving according to hazards, that link doesn't exist in real-life.

Cheers
Peter

You don't have to have your eyes glued to the speedo to be affected by the speed kills policy though. I find I spend more time thinking about whether I am within the speed limit then I did before. Frankly it is a wasted thought as I should be thinking about the road ahead and what is going on around me rather than whether my speed matches that on the lollipops. I also find I am tending to drift along at whatever speed is on the signs rather than actually thinking about appropriate speed like one does in nsl limits.

Conversely I did go through a village where the residents had written a load of signs saying 'slow down' speed kills yada yada and I wondered how long it is before one of them gets run over as a driver was too busy reading their hand written signs

Oh yes. It's massive. The effects are absolutely everywhere.

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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