Road safety in Nordelph
Following a 'Speed Awareness Monitoring' exercise in the village last Summer, carried out on the Downham--Wisbech Road, we requested the detailed data from Norfolk County Council. This is the analysis of that data.
Scope
NCC supplied seven data sets, each corresponding to one monitoring period. All the data refers to morning traffic, generally between 8.00 and 10.30. Different sites were used in different exercises. We analysed just four of the data sets, totalling 290 paired recordings of entry and exit speeds -- we limited ourselves simply because the data was printed and not in machine-readable form, so the 580 entries had to be keyed in, and checked, by hand! We did however continue up to the point where adding new data was having no significant effect on the conclusions. You can download the raw data in spreadsheet form (not recommended) or a summary of the analysis from the website. Both these documents treat the four data sets separately -- for this analysis, we have combined all the data into a single 580-item set. It is not clear that this is statistically justified, as the individual data sets were collected at different times and in different places. However, this approach does make it possible to summarise the data fairly simply.
Entering the village
There is a 40 mph speed limit through the village. The average speed recorded on entry was 41.1 mph. It should be remembered that speedometers are only legally required to be accurate to 10% of the displayed speed, so that perfectly law-abiding motorists might record 44 mph at this point. No cause for concern here!
However, averages conceal a lot! In this case, they conceal the fact that the fastest vehicle entering the village clocked a breath-taking 77 mph!
It is well to be cautious at this point -- examining the raw data around this point reveals that six vehicles entered the village in a three-minute period at speeds of, respectively, 75, 48, 52, 76, 67 and 77 mph, and exited zero seconds later at identical speeds. This smacks of contaminated data that, since these are the first readings in that particular set, may well be due to setting up the equipment.
We may also see the effect of living in an agricultural area, prone to delays behind farm vehicles: the lowest recorded entry speed was a mere 4 mph!
What averages mostly conceal is how widely spread the data is -- ignoring the possibly 'unclean' data mentioned above, what can we deduce? The median entry speed overall is 41 mph, almost identical to the average. The median entry is the one with as many items above as below -- in other words, 50% of motorists entering the village exceeded the limit!
But again, we need to be cautious, and recognise that this would not be as much of a concern as a screaming headline might represent it if the majority of the data referred to drivers that were just a little bit over the limit. Digging deeper into the statistics, we can see that 83% of motorists entered at speeds less than 50 mph, and 98% at speeds less than 60 mph.
Leaving the village
We need to recognise that many motorists will slow down through the village in any case, and may well trigger a speed detector in the few seconds when they are still decelerating. Without knowing the siting of the detectors relative to the speed limit signs, we can't do much more than speculate here.
The mean exit speed, 31.9 mph, and the median 32 mph are again almost identical. Those arguing for a 30 mph limit through the village may take comfort from the fact that half the traffic travels out of the village at this speed anyway.
What about the spread here? This time, 83% of motorists left at a speed below 42 mph, effectively (remembering speedometers) within the speed limit of 40 mph; 98% did so below 52 mph.
The effects of SAM
Of course, this is data collected about drivers who were passing a display of their speed. It is reasonable to assume that some at least of them modified their behaviour as a result. It is this argument that is put forward by supporters of 'smiley face' devices such as the one in Welney. Did SAM make a difference?
The most dramatic individual speed reduction was of 43 mph. The least effective case was of a motorist who speeded up by 34 mph! These are cuiousities, however, and not to be trusted -- particularly as some are associated with potentially contaminated data.
On average, motorists passing through while SAM was watching decreased their speed by 9.2 mph. The median reduction, 8 mph, is rather lower. In other words, while more people
reduced speed by 8 mph than less, there were enough large reductions to raise the average.
In pictures
As a last summary of our data, here it is in bar chart form:
This is perhaps not the best display, but it shows many of the things described above. The data have been grouped into groups such as 30-39 mph. The column on the left is 'less than 10 mph', that on the right '70 or greater'. Entry speeds are in the front, exit behind.
If you look closely, you can see the anomalous 'upturns' at either end, where crawling or speeding vehicles distort the picture. Most striking -- and not bad evidence for the installation of a permananent speed display facility -- is the shift from the most common exit speed (40-49 mph) down one category to the most common exit speed of 30-39 mph!
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