Sunday, 12 September 2010

Accidental Statistics

JERSEY'S most dangerous roads have been identified. At least 1,320 motorists and 240 pedestrians were injured in accidents on the Island's roads between January 2006 and the end of December 2009. But according to figures compiled by the Jersey Road Safety Panel, many of the smashes happened at a small number of accident blackspots. The Island's most dangerous road is St Aubin's Road, which saw 206 accidents between 2006 and 2009. Victoria Avenue is the second-most hazardous, with 173 recorded accidents during the same period. (1)

There are lies, damm lies - and of course, road statistics. What the statistics really do not say is how "dangerous" the roads really are. The roads in question are very high capacity roads, trunk roads, that convey the bulk of the traffic in and out of St Helier. Exactly the same kind of pattern emerges in a report on dangerous roads in Scotland. A road to Argyll is identified as "Scotland's most dangerous road", but it is also clear from the report that it is a key route and therefore has a high volume of traffic. Here is the report in question:

From Argyll's point of view, the area is now the known host of Scotland;s most dangerous road. The 14-mile stretch of the A819 between Inveraray and Dalmally in Argyll has been ranked at this level, with 12 crashes causing deaths or serious injuries from 2006-2008. This score is nearly three-quarters more than the total over the previous three years. Commenting today, Jamie McGrigor says: 'The Road Safety Foundation's report is yet more evidence of the need to improve the safety on the A819, something I have called for many times in the past. 'This is a key route in Argyll, both for local residents, businesses and the many tourists and visitors to the area. Everything possible must be done to reduce the accident risk on this road.

The failing in the statistics is in not correlating traffic accidents against traffic volume. If a road has a high volume of traffic, and the number of accidents are fairly evenly distributed across all the islands roads, then those with the highest volume will emerge as the most dangerous roads. But it might in fact be the case that some smaller roads see a relatively high volume of accidents over the volume of traffic passing through them -- these then would be the true dangerous roads - because it would be clear that they had a significantly higher number of accidents than perhaps roads with numerically greater accidents but a greater volume of traffic.

This is not immediately apparent, and of course the newspaper article simply reports the statistics without trying to make that much sense of them. It reports that there are more accidents when wet weather occurs after a long dry spell and more accidents may occur on the so-called more dangerous roads at particular junctions or "blackspots". What they do not do is to make the case for his judging those roads to be the most dangerous roads in the island, despite their naming the roads as such.

How the statistics play out can be seen in a well-known fallacy which arises from presenting figures without any proper interpretation. It is well known that more accidents occur in clear weather with good visibility than when there is a thick mist obscuring the driver's vision. Based purely upon numbers therefore it would seem that it would be more dangerous to be driving when it is bright and sunny than when it is thick fog. This shows very clearly the mistakes that can be made from purely relying on numbers alone. The fallacy occurs in not spotting that fog is a relatively rare condition compared to good visibility. The key here is to divide the accident statistics by the number of hours of clear weather and the number of hours of foggy weather -- this will show that accidents in fog are disproportionately higher.

When just dealing with numbers of accidents along particular roads it is easy to be seduced into thinking that the statistics are simple -- but just as with fog distorting the real import of the figures, so also traffic volume can hide which roads are really the most dangerous roads in the Island. In this way, the article by the JEP presents not just accident blackspots, but also illustrates - unintentionally - statistical blind spots.

Other factors which should be considered are (1) weather - for example, the variation in accident figures in total on the Islands roads are partly going to be effected by weather - how many times rain comes after long dry spells, how many times roads are icy, and (2) the number of cars on Islands roads. The latter, may indeed, be responsible in part from the overall increase in the figures since 2006, as more cars will certainly mean more accidents even if the chances of accidents remain the same.

What is just as dangerous as "the most dangerous roads" are the hidden factors which may make up the accident figures, and the impression of the JEP gives - that the increase is mostly due to drivers being more careless - is not as robust a suggestion as it first seems. That is not to say that care should not be taken in poor conditions, but it does mean that the ability of drivers cannot be inferred from the figures as they stand.


No comments: