For more than sixty years, blood alcohol concentration (BAC) limits have been used to define impaired driving (DWI) offences in terms of the amount of alcohol in the driver’s blood, breath or other bodily fluid. Defining DWI offences in terms of BAC has served to simplify the process of identifying and convicting offenders. The ultimate purpose in doing so, however, has been to reduce alcohol-related deaths and injuries. In this context, the selection of the most appropriate and effective BAC limit has often been a contentious and inherently political issue. The debate surrounding the appropriate BAC limit continues today. Indeed, the interest and concern that prompted the preparation of this report relates to the debate over whether Canada should lower the BAC limit in the Criminal Code of Canada from 80 mg/dL to 50 mg/dL. Canada has had a BAC limit of 80 mg/dL in the Criminal Code since 1969. Canada is, however, somewhat unique internationally in that all provinces, except Quebec, have a lower BAC limit of 50 mg/dL (40 mg/dL in Saskatchewan) which authorises police to issue 12- or 24-hour suspensions to drivers with BACs that exceed this limit. In this context, if a 50 mg/dL BAC limit were introduced in the Criminal Code, it would not reduce the threshold BAC at which sanctions can occur for most drinking drivers in Canada. It would primarily serve to lower the limit at which criminal sanctions are imposed.Nevertheless, there have been repeated calls over the past two decades to lower the BAC limit in the Criminal Code from 80 mg/dL to 50 mg/dL. Each time this has been considered, it has been rejected, most recently in 1999. Nevertheless, legislators and interest groups return to the issue of a lower BAC limit, partly because of its apparent simplicity, straightforward rationale, and perceived effectiveness. The primary purpose of this report is to contribute to the ongoing discussion about lower BAC limits through an assessment of the potential safety impact of a change in the BAC limit. The intent is to provide policymakers and other interested parties with a critical review of the existing evidence on the impact of lowering the legal BAC limit. The goal is to provide a thorough and objective assessment of the existing evidence and to consider the extent to which redefining criminal impaired driving behaviour by lowering the BAC limit in the Criminal Code of Canada might reasonably be expected to reduce the magnitude of the alcohol-crash problem in Canada. The main body of the report provides a critical review of research studies that have evaluated the traffic safety impact of a reduction of the BAC limit. Our primary interest was in studies that evaluated the impact of a change in the BAC limit from 80 mg/dL to 50 mg/dL, since this is the issue being considered in Canada. However, because of their relevance to the general issue of reducing the BAC limit, studies that examined other changes to BAC limits – i.e., from 100 to 80 mg/dL and 50 to 20 mg/dL – were also reviewed. The studies reviewed were evaluations of changes in BAC limits in the United States, Sweden, and Australia. As noted above, in conducting the review, it was evident that none of the changes in BAC laws in these countries were necessarily similar to what would occur in Canada because there is nothing comparable to moving the existing 50 mg/dL limit from highway traffic law to criminal law. Nevertheless, it is important and informative to examine the experience of other jurisdictions as a means to assess the potential safety impact of changes in BAC limits. As in any critical review of the literature, it is also recognised that no study is without shortcomings of some sort. The evaluation of road safety policies and programs can be challenging and often limited by the type and quality of data available, the researcher’s ability to exercise control over and/or account for extraneous events, and the difficulty in obtaining adequate comparison groups. Although this is a reality of applied research, such factors impact its validity and consequently the conclusions that can be derived from it. Many – in fact, most – of the studies we examined interpreted the findings as showing a beneficial impact from lowering the BAC limit. However, a critical review of these studies revealed that many are limited by weak research designs, dependent measures with incomplete, indirect or no information about alcohol involvement in crashes, and/or overstated conclusions. The evidence is further compromised by the coincident introduction of other laws and programs targeted at drinking and driving. The presence of other coincident laws and programs is a source of confounding that makes it difficult to determine which measure caused the change in alcohol-related collisions, or what weight to assign the contribution of various measures. Our critical review of such studies provides a less optimistic view of the impact of lowering the BAC limit than is contained in the conclusions of those studies. Even methodologically rigorous studies reveal inconsistent findings – some showed a beneficial impact, some found mixed results, and others have reported no beneficial effects. In conclusion, our review of the evaluation literature failed to provide strong, consistent and unqualified support for lowering BAC limits. At best, the results are mixed and the methodological weaknesses in the studies raise questions about the robustness and veracity of the evidence. (A)
Samenvatting