INCONSISTENCY OF SURROGATE MEASURES OF ALCOHOL-RELATED ACCIDENTS IN SOUTH AUSTRALIA

Transcription

1 INCONSISTENCY OF SURROGATE MEASURES OF ALCOHOL-RELATED ACCIDENTS IN SOUTH AUSTRALIA MARK J. KING ROAD SAFETY DIVISION, SOUTH AUSTRALIAN DEPARTMENT OF TRANSPORT SUMMARY Three major changes in drink driving enforcement have occurred in South Australia since The effect of these changes on a number of surrogate measures of alcohol involvement in accidents were investigated. The surrogates included alcohol involvement of driver fatalities, and combinations of casualty, serious casualty, single vehicle and nighttime accidents. Data from previous studies were also cited. It was found that relationships between surrogate measures were inconsistent, and incompatible with assumptions about drink driving levels and related accidents. It was concluded that until these effects are understood the use of surrogate measures should be treated with caution. INTRODUCTION Random breath testing (RBT) was introduced in South Australia in The legal blood alcohol concentration (BAC) limit is 0.08gms/100ml. Learner and probationary drivers (first year of licensure) have been subject to zero BAC restrictions since 1985, and 0.05 BAC prior to this. South Australia is large (988,377 square kilometres or 379,760 square miles), with the majority of the population (1 million) residing in the capital city, Adelaide. The difference in population distribution between Adelaide and the rest of South Australia means that RBT is relatively easy to introduce, control and evaluate in the metropolitan area, but presents significant logistical problems in the rural area. For this reason, and because some of the data which will be used is only available for Adelaide, the ensuing discussion will be confined to RBT in the metropolitan area. There have been three major RBT events in South Australia. RBT was introduced on October 15, 1981 at a relatively low level, with the equivalent of one in ten licensed drivers being tested each year. On March 31, 1983 the number of RBT units in the metropolitan area was doubled to four units by reorganising (but not increasing) resources, and the testing intensity increased to the equivalent of one in six licensed drivers per year. On April 16, 1987 there were significant changes and increases in operation which led to a large increase in the potential number of RBT units and to the doubling of testing to the equivalent of one in three licensed drivers per year. All changes were accompanied by publicity, with the most intense campaign occurring in 1987, and the least intense in The changes in enforcement at each stage are described elsewhere ^1). There were no other important changes in this period. 578

2 It would be expected that the introduction or increasing of RBT should result firstly in a reduction in drink driving, and ultimately in a reduction in the incidence of alcohol-related accidents. Road users who are killed, or are treated at or admitted to hospital as the result of an accident, must have a blood test for alcohol. There are some exclusions for practical reasons, but casualty BAC data should provide a good measure of the effectiveness of RBT in reducing accidents. Unfortunately, such information is not readily available for casualty accident data prior to As a result, the evaluations of RBT which have been carried out in South Australia have made use of surrogate measures of alcohol-related casualty accidents. The only one of these which includes BAC data is driver fatalities. Other surrogates used include total driver fatalities, and combinations of casualty accident categories, single vehicle casualty accident categories, and nighttime casualty accident categories. The purpose of this paper is to compare various surrogate measures of alcohol involvement in accidents. Since these measures have frequently been used to evaluate RBT it was decided to investigate their relative responses to the major RBT events mentioned. METHOD The effects of each change in RBT enforcement were measured using surrogate measures available from the Department of Transport's accident database. The results of previous evaluations are cited where relevant. These include roadside surveys of drink driving conducted by the National Health and Medical Research Council's Road Accident Research Unit in January - May 1981, and February - June 1982, 1983 and The survey methodology is described in detail elsewhere ^2). In general only data were available, but fatality data was available from 1975 until June Because of seasonal variation in drink driving, before and after data in multiples of 12 months were desirable, but where necessary (see below) comparable quarters of data were examined. In the final quarter of 1985 there was a change in fourth quarter accident patterns in South Australia which appears to have been sustained. This was taken into account in selecting before and after periods. Table 1 lists the surrogate measures used. The before and after periods for analyses, given the above, were: 1981 change - Fatalities: October September 1981 (before), October September 1982 (after); casualty accidents: January - October 1981 (before), January - October 1982 (after) change - April March 1983 (before), March April 1985 (after) change - Fatalities: April March 1987 (before), April March 1989 (after); casualty accidents: April March 1987 (before), April March 1988 (after). These periods were maxima, subject to the following. Before data (multiples of 579

3 12 months) were compared with 12 months after data (except some analyses for 1981). If a significant decrease was observed, a further 12 months after data was combined with the first, and the tests were performed again. If no significant decrease occurred in the first analyses, three months of after data was compared with corresponding quarters in the before period, since a shortlived change in the measure may have occurred. The surrogate measures include proportions, tested using a proportion test, and frequencies, tested using the chi-square test. Expected values for frequency data are based only on the before period, and therefore ignore long term trends. RESULTS Significant results (p <0.05) are reported in bold type in Table 1, together with the before and after periods to which they apply. Nonsignificant results are reported only for the 12 month after period. A summary of the results for each change in RBT, including reference to roadside survey results from previous studies ^1,2), is as follows: October 1981 Analyses were limited since data prior to 1981 were not available. This meant that the three months immediately after the introduction of RBT were not examined. The only significant results found were decreases in the proportion and number of serious casualty accidents. A previous evaluation ^2) used a different definition for casualty accidents but included earlier accident data, and found a short-term effect of RBT on several measures. Roadside surveys were conducted in Adelaide in January - May 1981 and February - June 1982 and Even though the surveys did not cover the months immediately before and after the introduction of RBT, a nonsignificant decrease was observed for the proportion of drivers with a BAC of 0.08 or above in The proportion of drivers above zero in 1982 decreased significantly. It is of interest that fatal BAC data (available from 1975) and data based on combinations of alcohol-related factors tend not to show any effect, despite the fact that the roadside survey data showed a reduction in drink driving well into April 1983 The roadside survey results, which span the period February - June 1983, show that the level of drink driving in the period returned to the same level as in However, since the change in enforcement took place in the middle of this period, it is not possible to state whether or not drink driving levels changed after March 31. The 1983 change resulted in a sustained reduction in the number but not the proportion of driver fatalities over the legal limit. Total casualty accidents and the number which occurred at night both exhibited a short term decrease, proportion of serious casualty accidents which occurred at night decreased for at least 24 months. None of the measures which were combinations of alcoholrelated variables indicated an effect despite the decrease in alcohol involvement of drivers killed. April 1987 The roadside survey conducted in February - June 1987 around the doubling of 580

4 RBT in April 1987 was the first such survey to be conducted specifically to measure drink driving immediately before and after a change in enforcement. As has been reported elsewhere ^1), there was a large decrease in the proportion of drivers over the legal limit. This survey also differed from previous surveys in being conducted over a late night period only (10pm - 3am rather than 5pm - 3am). However, the change in drink driving did not translate into a reduction in either the proportion or number of alcohol involved driver fatalities, although total driver fatalities decreased by 25% in the 24 months following the change compared with the 24 months before. Casualty accidents, serious casualty accidents and single vehicle casualty accidents all decreased by 11% in the 12 months after the change. Larger changes would be expected to occur among single vehicle and serious casualty accidents than among total casualty accidents if the change in drink driving caused these changes, so that it is likely that the drop in these categories is attributable to other causes. The fact that the drop in driver fatalities is twice that among casualty accidents suggests that the fatality category has experienced an effect of increased RBT over and above any trend for a decrease in accents across the board. The number of night time casualty accidents similarly reduced by more than 11% in all categories except for single vehicle serious casualty accidents. The latter omission may be due to small numbers, since a nonsignificant 31% reduction occurred over 3 months. In general, the decrease in absolute number of night time accidents was short-lived (present after 3 months, but not after 12 months) and increased as more combinations of alcohol-related accidents variable were used. The proportion of nighttime accidents decreased in all categories, although the decrease tended to be short-lived. With the use of more alcohol-related accident variables the proportion tended to be higher initially and to evidence a larger decrease. DISCUSSION It would be expected that a decrease in drink driving is necessary for a decrease in alcohol-related accidents. However, the only decrease in alcohol involvement in driver fatalities occurred when there was no change in drink driving as measured by roadside survey. It is not suprising that the proportion of driver fatalities over the limit do not exhibit changes, because of small numbers. However, the nonsignificant changes were not as expected. Further, changes in alcohol-related accidents should be more likely to be detected among surrogate measures which combine several alcohol-related accident variables. This did not occur, except for the 1987 change. (It is worth noting that many nonsignificant results in the earlier years cannot simply be accounted for by a lack of statistical power, since the changes were generally small and sometimes in the opposite direction to that hypothesised.) The most important variables in the 1987 change were nighttime accidents and total driver fatalities. Changes in nighttime accidents figured in the 1981 and 1983 changes, but this was not consistent. Each period showed changes in some measures, but no measure changed in all periods. This may be understandable for 1981 and 1983, but still means that selection of a single surrogate measure would be unwise. There are possible explanations for some of these changes, but no explanation which explains all results found. Given the inconsistencies identified, it is suggested that conclusions made about alcohol-related accidents on the basis of 581

5 Surrogate measures should be treated with caution. The relationship between drink driving, alcohol-related accidents and other accident types appears not to be understood as well as is generally thought. REFERENCES 1. King, M. J. (1988) Random breath testing operation and effectiveness in (Road Safety Division Report Series 7/88). Adelaide, South Australia : Road Safety Division, Department of Transport. 2. McLean, A. J., Clark, M. S., Dorsch, M. M., Holubowycz, 0. T., and McCaul, K. A. (1984). Random breath testing in South Australia : effects on drink-driving. accidents and casualties. Adelaide, South Australia : NH & MRC Road Accident Research Unit, University of Adelaide. TABLE 1: SI IRROGATF. MEASURES AND RESUI TS (SIGNIFICANT IF P<0.05. IN BOLD TYPE) OF ACCIDENT ANALYSES AND LENGTH OF BEFORE/AFTER PERIODS (MONTHS. IN BRACKETS) MEASURES PROPORTIONS OF: 1. D killed 8 tested who were Al 2. SVSC accidents which were NT 3. SVC accidents which were SC 4. SVC accidents which were NT 5. SC accidents which were NT 6. SC accidents which were SV 7. C accidents which were SC 8. C accidents which were NT 9. C accidents which were SV KEY D - drivers and motorcyclists Al - alcohol involved (BAC 0.08 or above) NT nighttime (10 pm-3am) C - casualty (most severe injury required at least hospital treatment) SC - serious casualty (most severe injury required at least hospital admission) SV - single vehicle (excludes pedestrian, includes hit parked vehicle) SVC - single vehicle casualty SVSC - single vehicle serious casualty NUMBERS OF: 10. D killed & tested who were A l (approx. 20 p.a.) 11. D killed & tested (approx. 50 p.a., >90% tested) 12. SVSC accidents which were NT (approx. 90 p.a.) 13. SVC accidents which were NT (approx. 190 p.a.) 14. SC accidents which were NT (approx. 250 p.a.) 15. SC accidents which were SV (approx. 250 p.a.) 16. C accidents which were SV (approx. 600 p.a.) 17. C accidents which were NT (approx. 600 p.a.) 18. C accidents which were SC (approx p.a.) 19. C accidents (approx p.a.) RESULTS (SIGNIFICANT RESULTS IN BOLD TYPE) OCTOBER MARCH APRIL BEFORE AFTER BEFORE AFTER BEFORE AFT! PROPORTION 1. 39%(72) 50%(12) 49%(12) 35%(12) 35%(24) 42%(12) 2. 31%(9) 31 %(9) 32%(12) 32%(12) 40%(2x3) 26%(3) 3. 53%(9) 50%(9) 46%(12) 47%(12) 38%(?4) 44%(12) 4. 31%(9) 30%(9) 31 %(12) 31 %(12) 35%(2x3) 26%(3) 5. 18%(9) 19%(9) 19%(12) 17%(12) 18%(2x3) 13%(3) 19%(12) 17%(24) 6. 18%(9) 18%(9) 18%(12) 16%(12) 17%(24) 19%(12) 7. 40%(9) 35%(9) 35%(12) 37%(12) 32%(24) 32%(12) 8. 14%(9) 15%(9) 15%(12) 14%(12) 14%(2x3) 11 %(3) 9. 14%(9) 13%(9) 13%(12) 13%(12) 14%(24) 14%(12) NUMBER (72) 24(12) 26(12) 16(12) 41(24) 19(12) 26(12) 38(24) (72) 48(12) 53(12) 46(12) 118(24) 45(12) 118(24) 89(24) (9) 61(9) 80(12) 81(12) 172(24) 88(12) (9) 120(9) 172(12) 167(12) 105(2x3) 40(3) (9) 205(9) 278(12) 260(12) 556(24) 228(12) (9) 196(9) 253(12) 256(12) 501(24) 252(12) (9) 395(9) 553(12) 544(12) 1303(24) 578(12) (9) 458(9) 166(3) 132(3) 1341(24) 562(12) (9) 1093(9) 1433(12) 1575(12) 3026(24) 1343(12) (9) 3082(9) 1069(3) 996(3) 9368(24) 4161(12) 582