Drink driving behaviour and its strategic implications in New Zealand

Transcription

1 Drink driving behaviour and its strategic implications in New Zealand M ichael D. Keall and W illiam J. Frith Land Transport Safety Authority, P. O. Box 2840 W ellington, New Zealand. INTRODUCTION In 1995 alcohol contributed to 32.3% of New Zealand s fatal traffic crashes and 18.1% of injury crashes (Land Transport Safety Authority, 1996). Although these proportions are large, the role of alcohol in crashes has decreased over the last seven years (Ibid). Alcohol consumption generally has also decreased (Statistics NZ, 1997). The term drink driving is used here to mean driving while over the legal breath alcohol limit. This is 400 Xg/L for drivers 20 years old and over, and 150 (Xg/L for drivers under 20 years. The lower limit for young drivers is a response to their greater crash risk at low alcohol levels (Mayhew, 1993). This paper uses information from a travel survey and two roadside alcohol surveys to study drink driving behaviour and comment on its implications for countermeasure strategy. THE NEW ZEALAND TRAVEL SURVEY, JULY JUNE In this survey (Jones and Phipps, 1992), personal interviews were used to collect detailed travel data for two specified travel days from a random sample of 8,719 people. All motor vehicle drivers were also asked the times and locations of their alcohol consumption during the two travel days. The amount and type of alcohol were not recorded. Males were more likely than females to drive soon after consuming alcohol. For males 2.7% of all driver trips began less than 1 hour after the end of a drinking session, while for females the figure was 1.3%. Alcohol related fatal accidents tend to occur at night (Land Transport Safety Authority, 1996), which is consistent with Travel Survey data indicating that nearly 20% of drivers who began their trips between 10pm and 3am had consumed alcohol less than an hour before they began driving

2 THE ROADSIDE ALCOHOL SURVEYS 1995 AND 1996 These voluntary surveys were held over late summer and autumn (February to May) of 1995 and 1996, on Friday and Saturday nights between 10pm and 2am (the main drinking times). Police Officers and at least one civilian interviewer stopped, breath tested and interviewed motorists passing randomly chosen sites for approximately two hours per site. In 1995, the Police stopped vehicles and directed them to the roadside where civilian interviewers breath tested and interviewed the drivers. In 1996, the Police stopped vehicles, breath tested and collected age and sex details from all drivers. A random sub-sample of drivers were interviewed by the civilian interviewers. In neither survey were breath alcohol results known to the interviewer as they were not displayed but stored directly in the device s memory. The sample size was 5,899 breath tests and interviews in In ,594 drivers were breath tested but only 1,309 drivers were interviewed. The 1995 survey covered population centres in a restricted geographical area covering about 57% of New Zealand s population. The 1996 survey, however, covered the entire country. Survey sites were randomly selected from roads with a speed limit of 50km/h using local maps and a database of all road sections. Sites were then visited to check their operational practicality. Cost considerations precluded using very low volume roads. The data were weighted to account for the following: variations in the proportions of traffic sampled; sampling probabilities applied to potential sites; the length of time each site was surveyed; how many times a site was used (normally once, or rarely twice). In 1995, 63 drivers (1.1% of drivers surveyed) refused the voluntary breath test and interview. 297 drivers (5% of the total sample) had difficulty blowing the 1.2 litres of breath required by the testing device for a valid sample. Imputation, described below, was used for these drivers. Despite the fact that the 1996 survey was also voluntary, there were only three outright refusals. The 1996 breath testing device (an Alcotech AR1005) was programmed to take a reading automatically on the third attempt if the driver s first two attempts failed to provide sufficient breath (a partial test). An experiment, in which volunteers at various breath alcohol levels (BrAC) deliberately blew insufficient air into the breath testing device, showed that the partial tests produce slightly lower readings, the bias increasing as breath volume decreased. The results, which were consistent with other studies (eg Bell and Flack, 1995), were used to adjust the partial test BrAC levels in the survey data. The estimated

3 proportion of drivers over the legal limit for these partial breath sample drivers was more than double the proportion for the full sample drivers. This information was used to impute levels of drink driving for the drivers who failed to complete a valid test in the 1995 survey. As sites were randomly sampled, a limited exposure measure could be derived: weighted counts of drivers passing the sites by category (eg drivers over the legal limit) were used to estimate the number of all such drivers passing sites in the areas surveyed. Estimates of '% passing sites, based on the weighted counts, appear below. The figure, '% of all over limit' is the estimated proportion of all drivers over the legal limit in the given category. Confidence limits used for the 1995 survey estimates (discussed in more detail in Keall, Frith and Perkins, 1996) derive from a cluster sampling variance formula (Cochran, 1977) and confidence limits for the 1996 survey from the random groups technique (described in Sarndel, Swensson and W retman, 1992). Design effects (Kish, 1965) of 4.1 and 2.8 were calculated for the 1995 and 1996 surveys respectively, the difference arising from a more efficient sample design in Many analyses of roadside surveys have made inappropriate use of simple random sampling variance formulae (effectively assuming a design effect of 1 ), thus dramatically increasing the chance of Type I errors. A G E O F D R IV ER In Figure 1 the solid line indicates the estimated percentage of drivers above the legal lim it by age group. The shaded area represents the 95% confidence interval for these estimates. Figure 1, which uses national data from the 1996 survey, indicates a high level of drink driving among teenage drivers. As the legal BrAC limit is much lower for teenagers, a dotted line indicates the percentage of teenage drivers over the adult limit (400 pg/l). Older drivers appear to be more likely to be over the adult limit than younger drivers

4 Figure 1: Estimated percentage over limit by age Percent of drivers 7% Age category This graph suggests that young drivers appear to be adhering to the adult limit rather than to the youth limit. The high level of drink driving among young drivers appears a serious problem when the proportions of all drivers on the road and of all drink drivers on the road are considered. Figure 2 shows (1996 data) that teenage drivers, represented by the exploded slices, constituted about 1 2 % of all drivers on the road during the main drinking times, and 26% of all drinking drivers. Thus this relatively small group, who drove less than 4% of total distance driven (Jones and Phipps, 1992) appear to travel disproportionately during the main drinking times and even more disproportionately while over their legal limit. Figure 2: Drivers on road and drinking drivers by age: 1996 drivers on road drinking drivers a B 60 plus

5 DIFFERENCES BETW EEN THE SEXES Table 1: 1996 roadside alcohol survey estimates by sex sex no. sampled % over lower 95% upper 95% % o f all % drivers legal limit confint confint over limit passing sites female % 0.7% 1.8 % 23% 36% male % 1.7% 2.9% 77% 64% Table 1 indicates that the estimated drink driving rate for males in 1996 was almost twice that of females. This is why, despite that fact that 64% of drivers on the road during the main drinking hours and days were male, an estimated 77% of drinking drivers were male. ESTIMATES BY START OF DRIVING TRIP AND URBANISATION Drivers were asked in the 1995 survey Which o f the follow ing places have you come fr o m T. Drivers coming from restaurant, other home/party and sports club/pub/hotel were more likely to be over the limit than other drivers (Table 2). Drivers coming from other home/party constituted 42% of all drink drivers, more than the combined total from commercial drinking establishments. Travel Survey data indicate that with decreasing urbanisation, other home/party becomes more preferred as a drinking venue prior to a driving trip. Both 1995 and 1996 alcohol survey data also indicate increased drink driving with decreasing urbanisation. The above travel survey finding is not apparent from the alcohol surveys. A probable explanation is that roadside alcohol surveys are less likely to stop drivers travelling home from drinking at a private dwelling in rural areas than such drivers in more urban settings. The roadside surveys tend to focus on arterial roads with a 50km/h speed limit

6 Table 2: Estimates by start of driving trip (1995 roadside alcohol survey) start o f trip no. sampled % over lower 95% upper 95% % o f all % passing legal limit confin t confin t over limit sites nightclub/other % 0.2 % 4.2% 1 1 % 24% other home/party % 2.7% 6.3% 42% 29% own home/work % 0.8 % 2.7% 15% 28% restaurant % 2.6 % 12.3% 2 2 % 1 0 % sports club/pub/hotel % 1.2 % 8.3% 1 0 % 8 % CHANGES IN DRINK DRIVING RATES BETWEEN 1995 AND 1996 Annual road fatalities in New Zealand dropped by 67 from 581 in 1995 to 514 in Preliminary analysis of crash data indicates a drop of 3.4 in the percentage of serious injury and fatal crashes involving alcohol. There appears also to be a drop in the drink driving rate. Comparisons may be made between the 1995 and 1996 roadside alcohol surveys, using 1996 data only from those areas also surveyed in Design and stratification changes meant that relatively few of the 1995 alcohol survey sites were used in 1996, so the variance of the estimators of change between 1995 and 1996 were relatively large. Nevertheless, the sizes of many of the changes were sufficient to be statistically significant. For areas surveyed in both 1995 and 1996, the overall drink driving rate fell from 3.1% to 2.5%. Statistically significant decreases in drink driving rates were found for the following classes of drivers using a 1-tailed z-test at the 10% level (P value in brackets): All drivers (P=0.10); Drivers after midnight (P=0.07); Male drivers (P=0.07); Drivers in the age ranges (P=0.04) and (P=0.07); Male drivers aged (P=0.03); Males driving after midnight (P=0.07); Drivers in provincial cities (population 10,000 to 100,000) (P=0.10). No other estimated changes - whether decreases or increases - were statistically significant. Figure 3 shows the estimated distribution of drivers above given BrAC levels derived from the 1995 and 1996 roadside surveys. There is clearly a shift towards lower BrAC levels from 1995 to

7 Figure 3: Estimated distribution of drivers above BrAC levels % over given level LnLOLninLnirjLotoinLnLOLOiDioininioioinLOiOLnLninLo LncOT-^j-h-ococDCDCNjLnco-T-'dT^-oc^cooicMLnco-^-rj-h' -i-t-i-c\jc\jc\]c\jcococo^f^-^-ir>loloir)cd<dtdh-h~h~ Breath alcohol levels STRATEGIC IM PLICATIONS New Zealand has of late had a drink driving deterrence strategy involving intensive random breath testing combined with hard-hitting and extensive advertising, broadly in line with the model used successfully in Victoria, Australia (Cameron et al, 1995). These results, and the downward direction of other indicators are supportive of the usefulness of this strategy. The following observations are germane to the planning of future deterrence strategies, More drink drivers originate from private homes than from commercial drinking venues. W ith decreasing urbanisation, drinking is more likely to occur in a private home, and overall drink driving increases. Drink driving is still very much a male activity. The youth limit is widely ignored. Older drivers appear more likely to exceed the adult limit than younger drivers. This is consistent with positive effects of alcohol related initiatives directed at young people and augurs well for the future as this group ages and passes its values to the next generation

8 R E FE R E N C E S Bell, C. M. and Flack, H. J, (1995) Examining variables associated with sampling for breath alcohol analysis. Proceedings 13th International Conference on Alcohol, Drugs and Traffic Safety, Road Accident Research Unit, University of Adelaide, Australia. Cameron, M., Newstead, S. and Vulcan, P. (1995). Analysis of reductions in Victorian road casualties, 1989 to Proceedings 17th ARRB Conference. Cochran, W. G (1977). Sampling Techniques. 3rd ed. (John W iley and Sons: New York.) Jones, W. R. and Phipps, P. D. (1992) New Zealand Household Travel Survey July 1989-June Traffic Research Report 43, W ellington, NZ: Safety Standards Branch, Land Transport Division, Ministry of Transport. Keall, M. D., Frith, W. J. and Perkins, W. A. (1996) The patterns o f alcohol use by New Zealand drivers during the main drinking hours: survey results and m ethodology. Proceedings combined 18th ARRB Conference and Transit NZ Land Transport Symposium. Kish, L. (1965). Survey Sampling. (John Wiley & Sons: New York.) Land Transport Safety Authority (1996) M otor Accidents in New Zealand 1995 Mayhew, D. R. (1985) Alcohol, age and risk of accident involvem ent. Proceedings 9th International Conference on Alcohol, Drugs and Traffic Safety, NHTSA. US Dept, of Transportation. Sarndal, C., Swensson, B. and Wretman, J. (1992) M odel assisted survey sampling. New York: Springer-Verlag. Statistics NZ (1997) Key Statistics: total alcohol available fo r consumption. D ISC LA IM E R This paper represents the views of the authors and does not necessarily represent those of the Land Transport Safety Authority