In Australia during 1996, almost 2,000 (1,973) persons died of injuries which were the result of a road traffic crash. This represents 11 fatalities per 100,000 population per year. In addition, the rate for serious casualties resulting from road crashes is typically at least ten times that of fatalities (Federal Office of Road Safety, 1994).
Young people are affected disproportionately, which is of special concern. In 1995, 38 per cent of all road traffic crash fatalities were under 25 years of age. More generally, with the exception of the first year of life, road trauma is the leading cause of death amongst individuals aged less than 25 years and one of the foremost causes amongst individuals less than 45 years (Australian Bureau of Statistics, 1995). For males, almost 16 per cent of years of potential life lost before age 65 is due to road trauma, which is more than the contributions of heart disease and cancer (Federal Office of Road Safety, 1992).
Vehicle occupants account for the majority of those fatally injured, 68 per cent in 1995, with pedestrians (20%), and then motorcyclists and pedal cyclists (10 and 2%, respectively) (Federal Office of Road Safety, 1996). The latter groups of road users are more vulnerable than vehicle occupants and are consequently proportionally over-represented among the fatal cases.
Excessive speed is reported to be an important contributory factor in many crashes. Analyses of a number of large data bases in the United States indicated that speeding or excessive speed contributed to around 12 per cent of all crashes reported to the police and to about one third of fatal crashes (Bowie and Walz, 1991). In Australia, excessive speed is an important factor in approximately 20 per cent of fatal crashes (Haworth and Rechnitzer, 1993) and speed is a probable or possible cause in 25 per cent of rural crashes (Armour and Cinquegrana, 1990). It has been argued that such figures are likely to under-estimate the role of speed in crashes because subtle effects, such as the amplification of other dangers in the traffic situation by relatively small increases in speed, are likely to be overlooked (Plowden and Hillman, 1984).
A large body of evidence suggests that there is a positive association between speed and the risk of crash involvement. This evidence includes the findings from case-control studies and from studies of fatality and casualty rates before and after changes to speed limits, and evidence from comparisons of fatality rates for countries with different maximum speed limits. Three case-control studies conducted in the United States more than 20 years ago attempted to elucidate this relationship, but the validity of the results and their interpretation have been questioned. In addition, it is not clear how these results might apply in Australia at the present time.
Quantitative knowledge of the relationship between speed and the risk of crash involvement would advance the understanding of the causes of road crashes. It would also make an important contribution to the debate about the appropriateness of speed limits and their enforcement. Research to this end has been recommended on a number of occasions (Cowley 1987; Victoria, Parliament, 1991; Fildes, Rumbold and Leening, 1991).
The main aim of this project is to quantify the relationship between travelling speed and the risk of involvement in an injury producing crash, for sober drivers in an urban setting. Using a case-control study design, speeds of vehicles involved in injury crashes are compared with speeds of vehicles not involved in crashes but travelling in the same direction, at the same location, time of day, day of week, and time of year.
The secondary aims of the project are to examine the effect of hypothetical speed reductions on this set of crashes and to explore the relationship between travelling speed and driver blood alcohol concentration.
A number of studies have investigated the relationship between speed and crash risk but most of them have had significant limitations. They are discussed in detail in the literature review section of this report.
The results of a pilot case-control study, conducted by the Road Accident Research Unit in 1994, of the association between travelling speed and the risk of involvement in a serious or fatal car crash showed that there appeared to be at least as high a crash risk involved in travelling more than 25 km/h above the 60 km/h speed limit as there was in driving with a blood alcohol concentration above 0.15 (Moore, Dolinis and Woodward, 1995). However, the pilot study did not contain enough cases to assess the effect on crash risk of travelling only slightly above the speed limit, nor did it attempt to address the possible confounding effects of alcohol impaired driving.
The present study attempted to deal with both of these limitations by collecting a larger number of cases to increase statistical power and by studying only sober case and control drivers to eliminate confounding by the effects of alcohol.
By looking at actual crashes and hypothesising different travelling speeds, some insight can be gained into the possible effects of lowering travel speeds. The present report thus includes a section in which travelling speeds are hypothetically varied.
It is well established that there is a positive association between driver blood alcohol concentration and risk of crash involvement (Borkenstein, et al., 1974; McLean, Holubowycz and Sandow, 1980), however, there is little research that explores the relationship between travelling speed and alcohol impairment. Both are known to be risk factors for road crashes but it is not clear how they relate to each other. The present study will attempt to quantify this relationship.
1.1 Aims of this Project
1.2 Background
1.3 Effects of Lowering Speeds
1.4 Alcohol and Speed