Road accidents and rainfall in a large Australian city

Abstract

We investigate the impact of rainfall on daily road accidents in the metropolitan area of Melbourne, Australia, over 1987–2002. Our analysis from several viewpoints of the accident count, which has been normalised for variation in traffic volume, indicated that the effect of rainfall is multifaceted. Owing to a large non-linear trend a subdivision into three epochs (1987–1991, 1992–1996 and 1997–2002) was made. Nominal daytime and nighttime as well as 3 h raw counts were available for the first two epochs only.

Generally, the effect of rainfall across the epochs shows a tendency for larger values in autumn with smaller values in spring. For the daily, daytime and nighttime cases there is an approximate 40% decrease in both the volume-normalised dry and wet means from the first to second epoch. Since the second epoch is wetter than the first, and both dry and wet cases are affected in a similar way, then it appears that a non-weather influence is at work. It is suggested that law enforcement measures may be largely responsible.

We obtained a conservative estimate of relative risk of an accident in wet conditions based on a matched-pair analysis of 3 h dry and wet periods over the first two epochs (1987–1996). As with other studies we find that the risk is greater than unity in almost all cases suggesting that the presence of rainfall consistently represents a driving hazard.

Rainfall occurring after a dry spell has an enhanced effect on the volume-normalised accident count as the spell duration increases. The effect of dry spells is more clearly described when broken down by rain class. Generally, there is an increase in the impact of a dry spell when it first rains as the spell duration and rainfall amount increase.

Introduction

There is a perception in the community that precipitation is a road traffic hazard. A number of studies show that precipitation in the form of rain and snow generally results in more accidents compared with dry conditions (Codling, 1974, Satterthwaite, 1976, Sherretz and Farhar, 1978, Brodsky and Hakkert, 1988, Fridstrøm et al., 1995, Levine et al., 1995, Changnon, 1996, Andreescu and Frost, 1998, Edwards, 1999, Eisenberg, 2004). An early study for Melbourne, Australia by Foldvary and Ashton (1962) found that 20% of accidents were rain-related. Palutikof (1991) observed that 16% of road accidents in built-up areas of Great Britain occurred when it was raining. Hit-object collisions and those involving multiple vehicles that are associated with lane-changing actions are more likely on wet roads (Golob and Recker, 2003). A summary of many of these studies is given in Eisenberg (2004). The findings in them are often difficult to compare since they are based on different accident or injury types, different time periods within a day and may or may not involve an adjustment for traffic volume. Some are based on daily fatal or serious accidents, while others include damage-only accidents. For instance, Keay and Simmonds (2005) found in Melbourne, Australia, that rainfall increased the volume-normalised count for injury accidents by 2.4, 1.9 and 5.2% relative to the daily, daytime and nighttime dry mean, respectively. Furthermore, there is some evidence that wet or snowy weather, particularly if coupled with severe storms, can deter motorists from venturing onto the road, i.e. there is a reduction in traffic volume (Knapp and Smithson, 2000). This reduction is generally of the order of a few percent (Codling, 1974, Hassan and Barker, 1999, Keay and Simmonds, 2005). The avoidance of potentially hazardous conditions may be due to a self-assessment by the road user or road weather alerts broadcast via the media (Khattak and DePalma, 1997, Hansen et al., 2001). There is also some evidence that the sudden onset of rainfall or snowfall may increase the accident count to a greater extent than other wet or snowy days. For instance, the first snowfall of winter (Fridstrøm et al., 1995) or the first wet day of a storm (Levine et al., 1995). Other studies (Brodsky and Hakkert, 1988, Eisenberg, 2004) have found that rainfall has an enhanced effect on the accident count after a dry spell, due to an accumulation of road grime creating a slippery road surface. Ivey et al. (1981) observed that the urban wet accident rate increased with the proportion of the time that the road was wet. Estimates of the relative risk of an accident in wet weather (Haghighi-Talab, 1973, Brodsky and Hakkert, 1988, Andrey et al., 2003) have emphasised the additional hazards associated with rainfall. A revealing aspect of rain-related risk was presented by Andrey and Yagar (1993) who found that the accident risk returned to normal as soon as the rainfall had ended, despite the lingering effects of wet road conditions.

This study examines the accidents in Melbourne, the capital city of the state of Victoria, Australia (37.8°S, 145.0°E) and its metropolitan area (MMA), with a population of 3.5 million (June 2002) and covering an area of 8800 km². We undertake a comprehensive investigation of several aspects of the influence of rainfall on road accidents in the MMA and will look at the differences between dry and wet days (and daytime and nighttime periods) as well as the effect of the amount of rainfall, i.e. rain classes. A measure of the relative risk of an accident in wet conditions will be examined and we will consider the effect of extended periods without rainfall, i.e. dry spells.

The data and methods are described in Sections 2 Data, 3 Methods. We will present the results with a discussion for the response of wet versus dry days (rain effect) and rainfall amount (rain class effect) in Section 4.1, the relative risk of an accident in Section 4.2 and the impact of dry spells in Section 4.3. Finally, our conclusions are given in Section 5.