The contribution of alcohol to night time crash risk and other risks of night driving

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Abstract

Many studies show that driving at night is more risky in terms of crash involvements per distance travelled than driving during the day. The reasons for this include the more prevalent use of alcohol by drivers at night, the effects of fatigue on the driving task and the risk associated with reduced visibility. Although the consumption of alcohol prior to driving occurs most commonly at night, drink-driving is not inherently a night time risk factor. This study decomposes the New Zealand risk of driving at night into risk associated with alcohol and risk associated with inherently night time factors. The overall risk associated with alcohol use by drivers was shown to decrease with increasing age for the most risky situation analysed (male drivers on weekend nights). Given the levels of drinking and driving on weekend nights, the overall effect of alcohol was shown to contribute almost half of weekend night time risk for drivers aged under 40 on lower volume roads, but to contribute little to overall risk on higher-volume roads, consistent with other research showing that higher-volume roads are not favoured by drinking drivers. Risk at night relative to risk during the day (excluding risk associated with drinking and driving) was shown to decrease with age. Roads with illumination at night are less risky at night relative to during the day than roads without illumination. The risks estimated in this paper reflect the behaviour of the road users studied and their prevalence on the roads under the conditions analysed.

Section snippets

Background

There are several well-researched factors that contribute to higher night time than daytime risks for drivers, including impaired vision, fatigue and alcohol (Saunders, 1997). Visibility at night is limited by the range of headlights and the fact that many important obstacles, like dark-clad pedestrians, animals, or unmarked vehicles, present little contrast with the environment (Leibowitz et al., 1998). Visibility is further impaired by glare from oncoming headlights (Theeuwes et al., 2002).

Method

The study was a case-control design of driver-kilometres (a kilometre of travel by a particular driver) by light four-wheeled vehicles (cars, vans and utility vehicles). The driver-kilometre was defined as a case if the driver had been involved in a crash on that road section. A control driver-kilometre consisted of a kilometre of road travel not involving a crash driven by a representative sample of New Zealand drivers. Thus, risk of crash involvement per distance travelled can be estimated by

Results

Estimates of risk of crash involvement per km were divided by estimates of risk excluding alcohol to show how alcohol (with usage levels of 1997/1998) affects the overall risk. The overall changes in risk due to alcohol, shown in Fig. 1, are most strongly affected by the numbers of drinking drivers in each age group and driving situation. For this reason, the daytime change in the estimated risk of injury crash involvement per kilometre driven was relatively small (represented by the dashes in

Discussion

A driving situation that is known to be associated with high night time risk of driver fatal injury is the carriage of multiple passengers (Keall et al., 2004). It would have been desirable to include this as a factor in the current study's model. However, the New Zealand traffic crash recording system has some ambiguity as to whether a driver is travelling with passengers or not, unless the passenger is injured or the crash is fatal. For this reason, although the corresponding data were

Conclusions

The overall impact of alcohol use on driver risk was shown to decrease with increasing age for the most risky situation analysed (male drivers on weekend nights). Given the levels of drinking and driving on weekend nights, the overall effect of alcohol was shown to contribute almost half of weekend night time risk for drivers aged under 40 on lower volume roads. There was estimated to be a smaller overall impact of alcohol-related risk on higher-volume roads at night or roads (including lower

Acknowledgements and disclaimer

The assistance and support of staff at the Land Transport New Zealand (formerly the LTSA) is gratefully acknowledged. Thanks are also due to Jim McKnight and two other anonymous referees for their considered and constructive comments on an earlier draft. This paper represents its authors’ views and not necessarily those of the Ministry of Transport.

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