Nigel Hawkes: Why Britain's road accident victims make car crash statistics

There are some things that it ought to be possible to count with reasonable precision: the number of people seriously injured on Britain's roads, for example.

But please don't ask the Department for Transport for the figures unless you've got an hour to spare and a supply of damp towels to cool your brow. It could be 26,000, 80,000, or 220,000, or pretty well any figure in between.

Such are the discrepancies in the published figures that the Select Committee on Transport in the last parliament asked the UK Statistics Authority to investigate just how hard the DfT had tried to reconcile them. The UKSA has launched just such an inquiry. It's overdue.

The lowest figure, 26,000, comes from police reports and has been steadily falling for a decade or more. Unfortunately, the numbers have diverged quite sharply from those recorded by the NHS for road accident victims admitted to hospital with serious injuries. The two figures matched in 1997-98, but in the latest set of figures the NHS data record 14,000 more serious injuries than do the police.

In addition, deaths from accidents are falling more slowly than injuries. It's hard to misrecord a death, so it is presumed they are counted accurately by the police. It's possible, of course, that the ratio of deaths to injuries has changed as cars are fitted with more and more safety features such as air bags. But it's also possible that serious injuries are being under-reported, particularly as they form the basis of the target set in 2000 for the DfT to reduce deaths and serious injuries by 40 per cent by 2010 against a baseline that is the average for 1994-98. There is another source of data, the National Travel Survey, which since 1997 has been asking a sample of people every year if they have been injured in a road accident. The NTS results suggest that three quarters of all injuries go unrecorded by the police, and for serious injuries the discrepancy is even greater. Rather than 26,000 serious injuries, the NTS suggests the annual number is 220,000.

"Is this not a very dramatic difference?" inquired Louise Ellman MP, the transport committee's chairman in the last parliament. Richard Aldritt of the UKSA, who was giving evidence, agreed. It is, in fact, an almost unbridgeable difference: a factor of 8.5 between the number of people who say they've been seriously injured, and the number of casualties with serious injuries recorded by the police.

The DfT statisticians have tried to make sense of these wildly different counts but their result is closer to guesswork than statistics. Their "best approximation" of serious injuries is 80,000, but it could be as few as 40,000 or as many as 120,000.

The strong suspicion is that the police are under-recording serious injuries and have not been discouraged from doing so by the DfT in order that it may meet its target, which it is due to do. Since the target is set by reference to the police-recorded injuries and no others, there's little question that by the letter of the law it has or will be achieved. But it's a meaningless triumph if it is reached as a result of progressively poorer recording. It means that, yet again, a target has perverted the accurate recording of important data.

There may be other explanations – exaggeration by respondents to the NTS, for example, or too small a sample. But the end result is that we have no clear idea at all how many serious injuries are caused by road traffic accidents in Britain. Bizarre but true.