50% increase in deaths planned for M4

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Globalti

Legendary Member
There was a bit on the BBC South Today about this. Oxfordshire CC switched off all their speed cameras last week but 'accidentally' left a few on. One camera recorded 62 speeders in a 5 day period, an 88% increase for the first week after the announcement. Another recorded 110, 18% more than the average. The council will not be prosecuting any of the drivers.

Just to blat some simple stats at this one.....

62 as an 88% increase implies that the mean number of speeders over this period is around 33. Using the poisson distribution (may not be completely accurate, but good for counting event type experiments) , there's about a 1 in 10,000 chance of this happening accidentally (99.99% upper confidence interval - even tails - on a Poisson with mu=33 is 61.5).

In the interests of fairness, the same analysis for the 18% increase yields about a 1 in 10 chance of the variation being purely statistical.

I'm not going to go all Paul Smith (RIP) about these fag-packet sums, but I can't imagine the council has any motivation to perform a long term study at ex-camera sites to establish some proper measurements.

Of course, we could just appeal to people's nice side by putting up a sign that says 'please' and possibly lights up, but I suspect that these were only effective because drivers were unsure whether there was a camera attached as well.

They switched off the cameras but the machines continued to record the speeds. I suspect that for many months the majority of cameras have not been working or have been destroyed; in certain parts of the country there's a vigorous campaign of sabotage, see: http://www.guardian.co.uk/uk/2003/sep/07/transport.ukcrime and on our Blackburn ring road there are three empty cameras, two of which even have no calibration marks painted on the road. I think there's a recognition that they have had their day and now that there is political interest in them, local authorities have lost the will to keep them working. I heard the piece on Oxfordshire and smelled a statistical "rat" straight away.
 

Bollo

Failed Tech Bro
Location
Winch
..... I heard the piece on Oxfordshire and smelled a statistical "rat" straight away.
Just to make it clear, I'm saying that the variation is extremely unlikely to be due to statistical effects only.

My little analysis is almost certainly wrong, or to be more precise much too crude. For example, there are too few sites, the timescale is too short to consider the effects of weather, the full weekly travel cycle, school holidays etc. What it would say to me is that there's a good reason to look, and to look for longer and over more sites. Also, the correlations between speeding and accident rates will be different at different sites. This isn't the same as saying speed isn't a factor in accident rates, just that it will be a more or less important factor depending on the location.

Ultimately, the decision to remove speed cameras (indirectly by removing funding) is purely political. It plays well with majority of voters, who are willing to accept a potential increase KSIs to indulge themselves on the roads. Any after-the-fact attempt at justification on safety grounds is just hypocritical bum gravy (ref Safespeed).
 

Glenn

Veteran
http://www.oxfordmail.co.uk/news/8339528.Speed_camera_data__misleading_/

SPEED camera bosses have been accused of misleading the public over claims more motorists are speeding since the cameras were switched off on August 1.

Last week, Thames Valley Safer Roads Partnership said a roadside camera on the A44 in Woodstock had seen an 18.3 per cent increase in speed offences since the switch-off compared to the average number caught this year.

At the same time a radar inside a second camera in Watlington Road, Cowley, registered an 88 per cent rise in offences when compared with figures in 2008 and 2009.

The partnership said the figures for 2010 were not available, as the camera had been switched off due to roadworks.

When the Oxford Mail requested 2008-9 data for the Woodstock camera – to make an equal comparison to the Watlington Road camera – the partnership said the figures were not readily to hand.

Now, the Oxford Mail having obtained the information, the figures actually show speed offences fell by four per cent when comparing the figures since the switch-off to offences in 2008-9.

All the county’s speed cameras were switched off after Oxfordshire County Council withdrew funding.

Within days the partnership released its statistics, alth-ough spokesman Dan Campsall warned: “These are very limited results from just two locations.

“However, if this is a trend that grows across Oxfordshire it is very worrying indeed.”

Woodstock town councillor and former mayor Peter Jay said: “(These figures) are lies, damn lies and statistics.

“It’s always wrong if anyone misuses figures and if a public authority misuses them it’s not only wrong but a disgrace.”

Mark McArthur Christie, the chairman of the Oxford group of the Institute of Advanced Motorists, said: “People have been very accepting of partnership statistics in the past and taken them at face value.

“If this gets people questioning camera stats that’s a good thing. The problem for the partnership is once people feel misled they feel reluctant to believe you a second time.”

Mr Campsall insisted there was no deliberate attempt to mislead the public about the figures.

“I don’t think there’s anything we have done that is disgraceful or lies and damn lies.

“As we have always maintained, these remain limited data sets and there is a great deal more study that will need to be undertaken to determine what the increased risk at decommissioned camera sites is.”

He said the inconsistency in the figures arose because the partnership were evaluating speeding data for a live broadcast on Radio 4 and wanted to get the most recent set of data prior to the decommissioning of cameras in 2010 to show the impact of the switch-off.

However due to road works the Watlington Road camera had not been in operation this year so the partnership has to use historic figures from 2008 and 2009 instead of the 2010 comparison for Woodstock.

The county council is doing its own survey of speed across the county.

Council leader Keith Mitchell said: “This does back up what I have been saying that we have got to wait a while to get some real information on this and when we do get it it has to be consistent with what we are comparing.

“I’m not an expert in this, I’m a finance person at heart and for us this decision has been about our finances and cutting the cameras or cutting children’s services.”
 
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