Sunday, 27 November 2011

20mph limits in Nottingham and Portsmouth -Pt1

Earlier this week, the Nottingham Post ran a series of articles on the Councils proposals to impose 20mph speed limits on some streets in Nottingham (Incidentally, the term “impose” is itself a loaded term (see here)).

The council has chosen Sherwood as the pilot area because it "includes everything throughout the city which could benefit from a 20mph limit, such as residential areas, steep streets, major bus routes, industrial areas and wide and narrow streets."

This is all well and good, but BFTF was interested in the evidence behind this proposed course of action. How have 20mph trials performed in the past? What criteria will the criteria use to decide whether the pilot study will be a success or not?

Let’s start with some terminology:
“20 mph Speed Limits” indicates the use of speed limit signs alone (without traffic calming measures)
“20 mph Zones” indicates indicates the use of signs and traffic calming measures.

Before moving on to what do the “pro” and “anti” groups say?
You can find out about the case against 20mpt limits at :

And the case in favour for 20mph limts at:

AAnd then looking at the evidence that 20mph limits work?
Much of the evidence for 20mph limits relates to the experience of Portsmouth, who implemented a 20mph speed limit (without additional traffic calming measures) in many of the city’s residential areas. A report on the effects of Portsmouth’s 20mph limits can be found here:
Interim Evaluation of the Implementation of 20 mph Speed Limits in Portsmouth
Final Report - September 2010

Before the scheme was implemented accidents stood at 183 per year, whereas afterwards they were at 142 per year, a 22%drop. During that period casualty numbers fell nationally by about 14% in comparable areas.

Vehicle speeds were also measured before and after the intorduction of the 20mph limits:

It has to be said that the report very carefully omits mentioning that the speeds of vehicles in the sites that had a speed “before” the trial of 20mph or lower actually INCREASED during the trial (BFTF estimates by perhaps 1-2mph).

Accident statistics were aslo presented "before" and "after":

It is important to note that some variability is to be expected when the number of annual accidents is low, so the increase in KSA figures is not necessarily significant. For a more extreme example of this, the report stated that the number of school children injured increased from 5 (3yrs prior to change) to 7 (2years after change). BFTF can see that Daily Mail headline now : “20mph speed limit results in 40% INCREASE in children being run over”.

If the Mail actually has someone on the staff who understands numbers then one can imagine them working it out on an annual basis “20mph speed limit results in 110% INCREASE in children being run over”. . .

Lastly, the report also compares the speed reductions achieved in Portsmouth with those in two other speed reduction schemes, in London and Hull. It is worth mentioning that these schemes included were much better funded than the Portsmouth scheme and included traffic calming measures:

So there you go, a bunch or relevant data all in one concise package. Shame the reporting in the mainstream media isn’t like this. . . .

BFTF will leave you to draw your own conclusions about the effectiveness of 20mph limits, as with most things in life, it’s complicated.

20mph limits in Nottingham and Portsmouth -Pt2

Following on from the post regarding 20mph speed limits, BFTF wanted to ask the council what their criteria would be for judging the trial in Sherwood a success, and also what the evidence was (in terms of accident reduction) for the claim in the Evening News that “"We have had considerable success with the 20mph zones around schools for at least five years – about 67 per cent of schools have them."

BFTF also sent an email to local mosques suggesting that this was an area where the Muslim community could find common ground with the wider society in campaigning for fewer road deaths (note that this does not nessesarily mean campaigning for 20mph limits). It might even be possible for Muslim organisations to be perceived as working for the common good and acting on the basis of the evidence available (It is sad fact that “Muslims” and “based on the evidence” are not statements that can generally be found in the same sentence).

Monday, 21 November 2011

Where would you like me to find drugs today?

BFTF is fascinated by the many ways in which eyewitness identification can be mistaken.

Even so, BFTF was surprised to read in a paper "Handler beliefs affect scent detection dog outcomes" about how trained sniffer dogs would "alert" more often if their handlers thought there were drugs or explosives present.

Where would you like me to find drugs today, officer?

The findings were . . . er. . .found during a study in which a series of handlers and sniffer dogs were asked to find drugs or explosives in a training area. Some of the handlers were told that the locations of the targets were marked with red paper (the dogs were told nothing - perhaps they were put in a soundproof booth with earmuffs playing gentle music?). What the handlers did not know was that there were no targets at all in the training area, so any "alerts" would be false

The handlers reported that dogs alerted more at marked locations than other locations, presumably because the dogs were picking up cues from their handlers.

The authors conclude that:

"This confirms that handler beliefs affect outcomes of scent detection dog deployments."

So, if you want to avoid sniffer dogs marking you as a priority for a cavity search, wear a suit !

Image Source : Wikipeida

Sunday, 13 November 2011

New research on Cod stocks in the Grand Banks

You may be aware of the story of the Cod Fisheries off the Canadian Grand Banks. These were some of the most productive cod fishing grounds in the world until overfishing resulted in a collapse in stocks in the early 1990's, at which point (to the sound of a stable door slamming shut) cod fishing was banned in the area to allow stocks to recover.

A recent paper by Kenneth Frank et al entitled "Transient dynamics of an altered large marine ecosystem", published in Nature, described the results of recent research into the Grand Banks marine ecosystem. The results are fascinating and provide reassurance that, given time, fish stocks to recover. We'll get to that in a moment, but first let us set the stage, so to speak.

The marine ecosystem in the Grand Banks can be broken down into three main parts.

At the top of the food chain are the large predator fish such as Cod and Haddock.

Below them are smaller fish such as Herring, Sand Lance and Capelin (no, BFTF had never heard of Sand Lance or Capelin before either). There are known as "forage fish" species.

And below these are the many plankton species.

Thus Cod feed on Herring and Herring feed on Plankton (it's a bit more complicated than this, obviously, but life is short so let's stick to the essence of the story).

Unsustainable fishing practices resulted in overfishing and a collapse of cod stocks in the early 1990's. With no Cod to keep numbers in check, the populations of the forage fish species exploded by some 900% (see graphic below).

But these high populations of forage fish were themselves too large to be sustained by the available plankton, so they in turn collapsed and entered into a "damped oscillation" of population peaks and troughs (see page 3 of the paper). The authors note that the period (time from peak to peak) is related to the life span of the forage fish and that
"Such eruptions followed by crashes involving fast growing, highly opportunistic species are known to occur in other ecosystems freed from predatory control"

Critically, part of the diet of the forage fish was the larvae of the large predator fish, which is why the stocks of Cod remained depressed for so long after the population collapse.

With the stocks of forage fish now moving back towards historical levels, it has finally been possible for Cod and Haddock stocks to recover.

The researchers note that, prior to the collapse, the dominant predator species was Cod, whereas the dominant species is now Haddock. Indeed, Haddock stocks are back up to historical levels, the stocks of Cod are still only 35% of those prior to the collapse. Only time will tell whether this change in the relative proportions of Cod and Haddock is a temporary or a permanent phenomena.

The authors comment that there are a number of factors that could still delay fish stock recovery (e.g. jelly fish blooms, the appearance of invasive species or eutrophication). Having said that, however, the authors have the encouraging view that
"These uncertainties notwithstanding, the answer to the critical question of whether or not such profound changes in the dynamics of large marine ecosystems are reversible seems to be ‘yes’."

You can see a press release about this research here