Alcohol, Travelling Speed and the Risk of Crash Involvement Jack McLean and Craig Kloeden Road Accident Research Unit, The University of Adelaide, Adelaide, Australia 5005 Abstract This paper compares the relationship between two factors that are known to affect the relative risk of involvement in a casualty crash: a driver's blood alcohol level and his or her choice of free travelling speed. It is concluded that measures which reduce travelling speeds are likely to be at least as effective in reducing the frequency of casualty crashes in Adelaide as measures which reduce drivers' blood alcohol levels. Introduction This paper reports on two case control studies: one, of the relationship between a driver's blood alcohol concentration (BAC) and the risk of involvement in a crash, and the other of the relationship between a driver's choice of free travelling speed and the risk of involvement in a casualty crash. Both studies were conducted in Adelaide, South Australia, by the Road Accident Research Unit (RARU) of the University of Adelaide. The study of the relationship between a driver's blood alcohol concentration and the risk of crash involvement yielded results which were similar to those obtained by Borkenstein et al in Grand Rapids, Michigan in 1963 (1). There has been no other study of the relationship between free travelling speed and the risk of involvement in a casualty crash in a metropolitan area. (One currently in progress in Montreal is due to be completed in 2003.) The results of the two case control studies reported here, which used similar methods and were conducted in the same metropolitan area, make it possible to compare the relative risks of crash involvement associated with a driver's BAC and choice of free travelling speed. Methods Alcohol and the Risk of Crash Involvement In 1979 the Road Accident Research Unit conducted a case control study of the relationship between a driver's blood alcohol concentration and risk of being involved in a crash relative to that of a sober driver (2, 3). It was based on an in-depth study of a representative sample of crashes to which an ambulance was called in metropolitan Adelaide (4). The BAC of a crash involved driver was either measured at the scene of the crash by the research team or the police or, for those who presented at a hospital for treatment of their injuries, by means of analysis of a blood sample which is legally required to be taken at hospital from all persons over 13 years of age thought to have been injured in a road traffic accident in South Australia. The post-crash interview with these drivers included questions relating to the route which they had followed, or had intended to follow had they not been involved in the crash. Control data was obtained at the same time of day and day of week as the crash at some point
on or near the case driver's route. Four drivers of about the same age and sex were breath tested when they stopped at a red traffic signal using a procedure developed for this purpose (5). The meter used at the crash scene for the cases and for all of the controls was the Lion Laboratories Alcolmeter PST. Travelling Speed and the Risk of Crash Involvement In order to estimate the relationship between travelling speed and the relative risk of crash involvement, a series of crashes was examined in detail in 1995 and 1996 to form the basis of a case control study (6). The case vehicles were passenger cars involved in casualty crashes in 60 km/h speed zones in the Adelaide metropolitan area (the speed limit throughout most of the metropolitan area is 60 km/h). The crashes were investigated at the scene by the Road Accident Research Unit and reconstructed using the latest computer aided crash reconstruction techniques. The case vehicles had a free travelling speed prior to the crash. A free travelling speed was defined as the steady speed of a vehicle moving freely along a mid-block section of road or with right of way through an intersection and not slowing to leave, or accelerating into, the road. The drivers of case vehicles were also required to have a zero measured BAC to exclude the effects of alcohol on the risk of being involved in a casualty crash. The 604 control vehicles (four per case) were passenger cars matched to the cases by location, direction of travel, time of day, and day of week. Their speeds were measured with a laser speed meter which looked like a video camera. The operator of the meter was typically some hundreds of meters beyond the location of the car when its speed was measured. In the early stages of the study, information on the control driver's BAC was obtained by "random" breath testing by the Police Breath Testing Section. As most of the crashes had been investigated during daylight hours, very few control drivers were found to have a positive BAC and those who did were at very low levels. Results Alcohol and the Risk of Crash Involvement Fifty nine (19.7%) of the 299 crash involved drivers had a positive BAC; the median value was 0.12 and the highest was 0.35. Almost two thirds of these drivers who had been drinking had a BAC above the legal limit for drivers which at that time was 0.08 g/100ml (it has been 0.05 since mid-1991). One hundred (8.1%) of the 1,096 control drivers had been drinking. Their median BAC was 0.03, the highest level was 0.18 and 18% of them were above the legal BAC limit. Subjective assessment indicated that the drivers who refused (4.4%) to cooperate were more likely to have been drinking than those who provided a breath sample. However, the refusals were also more likely to be replaced as controls by other drivers who also had been drinking (5.5% of the replacements were drinkers compared with 1.9% for all of the other controls). The relationship that was established between a driver's BAC and risk of involvement in a crash, relative to that for a sober driver, is shown in Table 1. At a BAC of 0.05 g/100ml the risk of crash involvement was found to be about 1.83 times greater than that for a sober driver. The 95% confidence limits on this estimate were 0.87 and 3.85. At a BAC of 0.10, the relative risk was approximately five times that of a sober driver.
Table 1: BAC of driver and the risk of involvement in a casualty 1 crash: Metropolitan Adelaide 1979 Nominal BAC 2 BAC range 2 cases controls Relative risk Lower Upper zero zero 240 1096 1.00 - - 0.02 0.01-0.03 8 53 0.69 0.32 1.47 0.05 0.04-0.06 10 25 1.83 0.87 3.85 0.08 0.07-0.09 7 10 3.20 1.20 8.48 0.12 0.10-0.14 14 9 7.10 3.04 16.6 0.15+ 0.15-0.35 20 3 30.4 8.97 103.3 Total 299 1196 1 Crashes to which an ambulance was called. 2 g/100ml 3 95% confidence limits of the estimated relative risk Travelling Speed and the Risk of Crash Involvement The research team attended 952 crashes at the scene. The study was based on 148 of these crashes. The reasons for excluding the other crashes are listed in Table 2. Table 2: Crashes attended and reasons for exclusion from the study Crashes attended Number of crashes Total number of crashes attended 952 Crashes excluded 804 No ambulance transport required 325 Case vehicle was not a car or car derivative 148 Case vehicle did not have a free travelling speed 148 Case vehicle doing illegal manoeuvre 26 Crash due to medical condition of driver 23 Site not in a 60 km/h zone 18 Not a vehicle accident 8 Case driver had a positive blood alcohol concentration 5 Case vehicle rolled over 4 Insufficient information for crash reconstruction 99 Crashes included in the case control study 148 Note: 3 crashes yielded 2 case vehicles each giving a total of 151 total cases Cars involved in the casualty crashes (the cases) were generally travelling faster than cars that were not involved in a crash (the controls): 68 percent of crash involved cars were exceeding the 60 km/h speed limit compared to 42 percent of those not involved in a crash. The difference was even greater at higher speeds: 14 percent of crash involved cars were travelling faster than 80 km/h compared to less than 1 percent of those not involved in a crash. The crash-involved cars were almost 10 times more likely to have been travelling faster than 70 km/h than were the non-crash-involved cars (29% vs 3%).
The relative risks of crash involvement for various speeds are shown in Table 3. It can be seen that the risk of crash involvement approximately doubles for each increase in travelling speed of 5 km/h above the 60 km/h speed limit. Table 3: 1995/6 Free travelling speed and the risk of involvement in a casualty 1 crash relative to travelling at 60 km/h in a 60 km/h speed limit zone: Metropolitan Adelaide Nominal speed 2 Speed range 2 cases controls Relative risk Lower Upper 35 33-37 0 4 0 - - 40 38-42 1 5 1.41 0.16 12.53 45 43-47 4 30 0.94 0.31 2.87 50 48-52 5 57 0.62 0.23 1.67 55 53-57 19 133 1.01 0.54 1.87 60 58-62 29 205 1.00 - - 65 63-67 36 127 2.00 1.17 3.43 70 68-72 20 34 4.16 2.12 8.17 75 73-77 9 6 10.60 3.52 31.98 80 78-82 9 2 31.81 6.55 154.56 85 83-87 8 1 56.55 6.82 468.77 88+ 88-147 11 0 infinite - - Total 151 604 1 At least one person transported from the scene of the crash by ambulance. 2 km/h 3 95% confidence limits of the estimated relative risk. Comparison of the Risks Associated with Alcohol and Speed Figure 1 compares the relative risks of casualty crash involvement at various blood alcohol levels and at various free travelling speeds in the Adelaide metropolitan area. It can be seen that a quite small increase in travelling speed above the speed limit results in an increase in the relative risk of casualty crash involvement that is comparable to that of an illegal blood alcohol level of 0.05. Each 5 km/h increase in travelling speed above 60 km/h increases the risk of involvement in a casualty crash by roughly the same amount as each increase in blood alcohol concentration of 0.05 g/100ml (Table 4). Table 4: Relative risks of involvement in a casualty crash for speed and alcohol Speed relative 1 risk BAC (g/100ml) 60 1.0 zero 1.0 65 2.0 0.05 1.8 70 4.2 0.08 3.2 75 10.6 0.12 7.1 80 31.8 0.21 30.4 Speed (km/h) BAC relative 2 risk 1 Relative to a sober driver travelling at the speed limit of 60 km/h. 2 Relative to driving with a zero BAC.
Figure 1: Relative risks of involvement in a casualty crash for speed and alcohol Relative Risk 40 30 20 10 0 60 65 70 Speed (km/h) 75 80 85 0.00 0.05 0.10 0.15 BAC (g/100ml) 0.20 0.25 Discussion Given that the crash risks associated with speeding and illegal drink driving are similar, and speeding is more common, why isn t speed listed as a cause of accidents more often than drink driving? The answer probably lies in the fact that it is a comparatively straightforward matter for a driver s blood alcohol concentration to be measured after an accident whereas the estimation of the travelling speed of a vehicle before the crash is rarely a straightforward matter. One consequence of this underestimation of the role of speed in accident causation is the marked disparity between the risks of involvement in a casualty crash and the penalties associated with speeding and illegal drink driving. Drink driving is seen as a serious problem requiring strong enforcement and severe penalties while speeding is generally seen as a comparatively minor road safety issue. The results presented here indicate that speeding needs to be taken just as seriously as drink-driving. Thus far we have compared the risks associated with speeding and drink driving. However, reducing travelling speeds can be expected to reduce the frequency of all crashes, including
those that are alcohol related. The reduction of travelling speeds, whether it be achieved by reducing speed limits and/or by the control of speeding, has the effect of making the driving task easier. If the driving task is made easier, drivers will make fewer mistakes, and that applies particularly to impaired drivers. If travelling speeds are reduced, the mistakes that are still made will be less likely to result in a crash, and those crashes that do occur will be less likely to result in injury or death. Acknowledgements We thank the Federal Office of Road Safety of the Australian Department of Transport and Regional Development for primary funding of both case control studies. Transport SA supplied some personnel to work on the speed case control study and the Australian National Health and Medical Research Council provided support through a Research Unit grant. We also thank the South Australia Police and the South Australian Ambulance Service for providing assistance. We also appreciate the support and contributions of the following people: Brian and Raymond McHenry (McHenry Software, Inc., Cary, North Carolina), Giulio Ponte, Lisa Wundersitz, Robert Baird and Matthew Baldock (RARU), Roland Earl and Roger Galbraith (Transport SA), and Chris Brooks and John Goldsworthy (Federal Office of Road Safety). References (1) Borkenstein RF, Crowther RF, Shumate RP, Zeil WB, Zylman R. The role of the drinking driver in traffic accidents. Department of Police Administration, Indiana University, Bloomington 1964. (2) McLean AJ, Holubowycz OT. Alcohol and the risk of accident involvement. In: Goldberg L, editor, Alcohol, Drugs and Traffic Safety, Proceedings of the Eighth International Conference on Alcohol, Drugs and Traffic Safety. Stockholm: Almqvist & Wiksell International, Stockholm, 1981, 1:113-123. (3) McLean AJ, Holubowycz OT, Sandow BL. Alcohol and crashes: identification of relevant factors in this association. Office of Road Safety, Commonwealth Department of Transport, Canberra 1980, CR 11. (4) McLean AJ, Aust HS, Brewer ND, Sandow BL. Adelaide in-depth accident study. Part 6: Car accidents. Road Accident Research Unit, University of Adelaide, Adelaide 1981. (5) Holubowycz OT, McLean AJ, McCaul KA. A new method of breath testing the general driving population. J Stud Alcohol 1991; 52:474-477. (6) Kloeden CN, McLean AJ, Moore VM, Ponte G. Travelling speed and the risk of crash involvement. Federal Office of Road Safety, Commonwealth Department of Transport and Regional Development, Canberra 1997, CR 172.