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External-to-Vehicle Driver Distraction
EXECUTIVE SUMMARY
1. This report presents the findings of a literature review of all available literature published in English since 1945 on the subject of external-to-vehicle driver distraction. The report was carried out by Human Factors Analysts Ltd. (HFAL) on behalf of the Scottish Executive between December 2002 and March 2003. The research consisted of three main elements. First, a general review of the literature pertaining to driver distraction. Second, a review of literature specifically concentrating on external-to-vehicle distraction. And finally, a review of literature pertaining to billboards and signs as an external distracter, in an attempt to discover whether there is evidence that billboards and signs are a contributory factor to road accidents.
Scientific Evidence on Driver Distraction.
2. There is a considerable body of scientific evidence to support the hypothesis that drivers can become distracted while they drive. The available literature suggests that there are two main elements to this: cognitive overload and cognitive underload. In terms of cognitive overload, the evidence suggests that when too much information is made available in certain situations the driver may become confused and have an inadequate amount of time to process the available information. In terms of cognitive underload, it seems that drivers (especially on long, boring trips), may cease to pay attention to the road, and may therefore become prone to distraction.
3. There are numerous studies that suggest that attentional/distraction problems are a major contributory factor to accidents. There are numerous individual differences/external factors influencing the likelihood of being distracted.
4. Information from accident databases suggest that external-to-vehicle driver distraction is a major contributory factor in road accidents. However, it is likely that external-to-driver distraction events are under-reported. This is because distraction is likely to be unconscious (and therefore unlikely to be consciously remembered and then put as a contributory factor on accident investigation forms), and also because there are social and legal pressures that make it less likely that 'distraction' will be admitted by drivers as being a contributory factor to the accident. Therefore it is almost certain that it is a larger problem than is commonly believed.
5. There seem to be two main features of distraction. As noted above, there is overwhelming evidence that drivers can become 'confused' by visual 'clutter' in some circumstances. This is caused by having to search through the whole 'visual field' in situations where the environment contains a great deal of distracting information. Statistical and experimental evidence suggests that this kind of distraction is most likely to occur at busy junctions.
6. There is also evidence that distraction can be associated with cognitive underload, and that drivers may become easily distracted in situations where there are low levels of information available in the external environment (for example motorways, possibly in conditions of low light). Drivers may then fixate on external lights/signs and drive off the road, or else may be particularly prone to being 'caught by surprise' by unexpected distracters. Statistical evidence suggests this may occur on sharp bends following long stretches of 'boring' road, amongst other places.
7. There are a number of correlation studies and experimental studies which demonstrate that drivers can be distracted by signs/billboards. None of these studies are conclusive, and there are a number of issues (especially concerning the 'correlation does not imply causation' rule with the statistical studies and 'ecological validity' with the laboratory studies) which mean that this finding should be viewed with a certain amount of caution. Nevertheless the case for arguing that visual 'clutter' at junctions (associated with billboards and signs) can lead to unsafe driving is very strong. However more research is needed on specific cases (especially in Scotland) to demonstrate the extent of the effect.
8. The evidence that advertising and signs can function as distracters in conditions of information 'underload' is less strong but still compelling. More research is therefore needed on this subject, given the rate at which outdoor advertising is proliferating.
9. It seems likely that young drivers (17-25) and older drivers (over 65) are particularly prone to distraction. However, older drivers seem to compensate for reduced driving ability by taking more care with their driving and driving less often, so this effect tends not to show up in accident statistics.
10. It must be stressed that despite the potential seriousness of external-to-vehicle distraction, most of the research has been carried out some considerable time ago (as far as we are aware, there has been only one research project on this topic since 1980). Moreover, almost all the research has been carried out in the United States or Australia. With the exception of two specific investigations of accident databases detailed in this report, there is (to the best of our knowledge) no published research on this subject carried out in Scotland.
Conclusions
11. The literature review found that driver distraction is a serious problem in road safety. It identified both internal-to-vehicle distraction and external-to-vehicle distraction as serious problems. External-to-vehicle distraction is likely to be under-reported, and seems likely to be a larger contributory factor than is commonly stated. It seems highly likely that billboards are a major contributory factor to such distraction incidents. However, almost all of the existing (available) studies were carried out before 1980, and none were specific to Scotland.
12. Implications for further work in this area include: carrying out specific research on what kinds of billboards and signs can create visual 'clutter' on junctions, showing what distraction effects this can have and the effect this has on safety; investigation of the effect of under arousal on driving ability and distraction; and more empirical research on specifically Scottish issues relating to distraction.
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