Road safety expert Idris Francis goes through the evidence

Bang on cue, within days of Oxfordshire's speed cameras being switched off, Thames Valley Safer Roads Partnership – at risk of closure if other Councils opt out – announced to a horrified Press that speeding had increased by up to 88% at six monitored sites. Their report for the whole of August, stated by Operations Manager Richard Owen to be "more significant", claims increases of up to 300% while warning that "Local authorities around the country should bear these results in mind if they are considering a similar approach to Oxfordshire as the deterrent effect of the housing alone is diminished by public announcements regarding their operational capacity".

It has been widely reported that Oxfordshire Council, apparently concerned by increased speeding and "risk", are considering switching the cameras back on in April – but just how "significant" really are the increases? Strangely, one might think, Professor Stephen Stradling, long involved in road safety, looked at the early numbers and commented on the Road Safety GB web site that, "a vanishingly small proportion" of passing motorists 'took advantage' of their new found freedom to drive above the speed limit. This is good news." So who's right?

The first thing to understand about TVSRP's data is that it was gathered only by counters triggered at the usual speed camera thresholds and can therefore tell us nothing about traffic volume, whether average speeds had risen, fallen or stayed the same, or anything about changes in excessive and therefore truly dangerous speeding. All it tells us is how many exceed the threshold. For these reasons and by any normal statistical standards, far from being "significant", the data is all-but meaningless. However let's take the numbers at face value:

Traffic volume may be estimated using the "two second rule" for vehicle spacing – maximum capacity of 30 vehicles per minute or 43,200 per day. No road operates at that capacity 24 hours a day but on the other hand some A roads in the TVSRP study might have 2 or more lanes, and many drivers follow at significantly less than two second intervals. It is therefore reasonable to assume that average traffic volume past the relevant cameras would have been of the order of 10,000 per day – i.e. 23% of capacity (arguably on the low side given that they are near busy Oxford).  Perhaps more, perhaps fewer, but the precise figure is not important:

Penalties per day in July, just before switch-off, totalled 100 for the six sites for a total of about 60,000 vehicles that passed by – a rate of 0.17%. In August, after switch-off, the counters triggered 408 times per day, a rate of 0.68%. As TSVRP is keen to warn everyone, this is indeed a four times increase – but it is also a fall in the proportion of drivers who do not break the speed limit only from 99.83% to 99.32% – no wonder Professor Stradling described it as "vanishingly small".

In any speed camera area the great majority of drivers drive just below, but close to the limit and that it would therefore not be in the least surprising if when the camera threat is removed they pay rather less attention to micro-managing their speeds by constant reference to speedometers, and rather more to what really matters, the view through their windscreens.

Is it in the least surprising that something like 1 in 200 drivers in these circumstances allow their speeds to rise by the odd few mph over the threshold, rather than just below it? If I am surprised it is only because that figure is so low.

I for one would far sooner be surrounded by large numbers of slightly faster drivers looking where they are going than drivers constantly looking at their speedometers – and indeed braking suddenly – to keep to an (essentially arbitrary) limit that is correct only in the same way as a stopped clock – occasionally and not for long. (In the 0.7 of a second it takes to look down, read a speedometer and look up again, the driver is "blind" for 30 feet even at 30mph and pro-rata at higher speeds). Could preventing this ever justify spending hundreds of thousands of pounds a year for Oxfordshire cameras alone? I suggest not.

Mr. Owen in his report made the extraordinary, but significant, statement that, "It will be several months before casualty data is available at the location to see if there is a correlation between the increase in offence rates and an increase in recorded injury collisions." I suggest in all seriousness, that if Mr. Owen really expects to be able to find a statistically meaningful correlation between "vanishingly small" speeding changes and a handful of injury collisions over a few months, he does not have the slightest understanding either of accidents or statistics and is in the wrong job.

My letter in the Daily Telegraph of Dec. 7th explains how Oxford Council and others, Partnerships and Acpo are now discussing plans to fund threatened cameras from profits from increasing numbers of Speed Awareness courses offered  as alternatives to fines and penalty points to drivers.

My letter also points out that this would take funding back to precisely the method the DfT decided in 2006 was not acceptable, because Partnerships “might be minded to look first for a road camera-based solution [to speeding] rather than a better and perhaps more cost-effective solution”. Which of course they and every police force continue to do despite the damning evidence I copied all of them in April 2007, and more recently here, that vehicle activated signs provide at least as much benefit as speed cameras at 2% of the cost – but with no need for Partnerships, enforcement system or cushy jobs for retired police officers in luxurious offices.

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