Road traffic crashes are a major contributor to the global burden of disease and injury. Approximately 1.35 million people die and a further 50 million are injured in crashes around the world each year. Road traffic crashes are the leading cause of death for young people aged 5 to 29 years (World Health Organization, 2018). Alcohol consumption is a leading contributor to crash incidence. Around 21.8% of fatal crashes globally are alcohol-involved, producing over 270,000 deaths each year (Vissers, Houwing, & Wegman, 2017). Preventive interventions to reduce the incidence of alcohol-involved crashes could therefore save many lives and avert millions of injuries each year around the world.

One universal prevention strategy to reduce alcohol-involved crashes that has a compelling theoretical foundation is sobriety checkpoints. In this intervention, a team of law enforcement officers establishes a roadside checkpoint at which passing drivers are stopped for a possible sobriety test. According to general deterrence theory, the approach affects crash incidence by raising the perceived risk of detection and arrest among all drivers within a population, which, in turn, increases the perceived individual costs of drunk-driving, leading to fewer people choosing to drive after drinking (Homel, 1993). Additional specific deterrent effects against drunk driving apply to the people who pass through the checkpoint; however, the proportion of all drivers who are tested at sobriety checkpoints and the arrest rates among those who are tested are very low (Voas, 2008), so specific deterrent effects are likely to be minimal (Watson & Freeman, 2007).

Sobriety checkpoints have very strong empirical support from studies around the world as a cost-effective and high-impact intervention to reduce alcohol-involved motor vehicle crashes (Community Preventive Services Task, 2014; Miller, Galbraith, & Lawrence, 1998). Multiple published reviews (Bergen et al., 2014; Elder et al., 2002; Erke, Goldenbeld, & Vaa, 2009; Shults et al., 2001) have concluded that crash rates are at least 8% lower in jurisdictions that use sobriety checkpoints compared to those that do not. These associations have been observed at a wide range of spatial scales—including within 2-mile buffers of checkpoint locations (Nunn & Newby, 2011), within cities (Voas, 2008), within counties (Lacey, Ferguson, Kelley-Baker, & Rider, 2006), and within states (Lacey & Jones, 2000)—and have been shown to be dose-responsive, in that programs that conduct more frequent checkpoints have greater reductions in crashes (Bergen et al., 2014). In addition to these studies that assess global associations for overall checkpoint programs, our previous analysis identified that individual checkpoint sites (couched within an overall program) are associated with reductions in alcohol-involved motor vehicle crashes for around one week (Morrison, Ferris, Wiebe, Peek-Asa, & Branas, 2019). In the absence of experimental research, observational studies provide convincing evidence that sobriety checkpoints are likely to be causally related to fewer subsequent crashes (National Academies of Sciences & Medicine, 2018). Rather than reproducing these many prior analyses, the imperative for research is now to identify optimal checkpoint configurations that maximize public health benefits while minimizing costs (Bergen et al., 2014). Such evidence will encourage and enable more law enforcement agencies around the world to use the intervention, thereby preventing more alcohol-involved crashes and reducing the global health burden.

The aim of this study is to assess the impact of sobriety checkpoint site configuration on alcohol-involved motor vehicle crash incidence. We selected the Australian state of Queensland as a study location because the Queensland Police Service has one of the most comprehensive sobriety checkpoint programs in the world, which included a process goal of achieving a ratio of one breath test per licensed driver in the state each year (Ferris et al., 2013). Queensland Police Service operates a program of random breath testing, which is a variant of sobriety checkpoints in which drivers can be stopped and subjected to a breath test without suspicion that they have been drinking. Full and highly resolved space-time data were available for all breath tests and all alcohol-involved motor vehicle crashes in the state, enabling assessment of the associations of interest at fine geographic and temporal scale. We focused on two elements of checkpoint configurations that are modifiable and important contributors to the total economic costs of checkpoint implementation: the duration that checkpoint sites are in operation, and the number of police officers involved.

In the 339 weeks between January 2012 and June 2018, there were 3,420 alcohol-involved motor vehicle crashes in Brisbane and Queensland Police Service conducted 5,654,715 breath tests. The hierarchical cluster analysis identified 2,069 checkpoints in which 550,415 (9.7%) of the breath tests were conducted. On average, there were 266.0 tests conducted per checkpoint (SD = 216.3). Checkpoints were an average of 286.3 minutes duration (SD = 261.3) and included 16.4 devices (SD = 13.7). Table 1 is a two-by-two table showing the checkpoint distribution according to dichotomized duration and size.

The weekly number of crashes followed an increasing trend which tapered around 2015. The weekly number of checkpoints followed a decreasing trend over time which also tapered around 2015 (Figure 2). Table 2 shows the descriptive statistics of all variables of interest in this analysis. On average, there were 10 crashes (SD = 3.5) and 6.3 checkpoints (SD = 3.9) per week, including 4.4 short checkpoints (SD = 3.3) and 1.9 long checkpoints (SD = 1.6); and 2.8 small checkpoints (SD = 1.9), and 3.5 large checkpoints (SD = 3.0). After aggregating within weeks, there was an average of 15.0 devices per checkpoint (SD = 6.9) and 241.8 tests per checkpoint (SD = 130.1), and checkpoints lasted an average of 292.9 minutes (SD = 65.5).

Table 3 shows the results of the GLARMA models. In Model 1, a 10-unit increase in the number of checkpoints per week had a weak association with the alcohol-involved crash rate for the one week later (RR_lag1 = 0.92; 95% CI: 0.84, 1.01) and the association was slightly attenuated two weeks later, though the 95% confidence intervals overlapped substantially (RR_lag2 = 0.96; 95% CI: 0.87, 1.05). In Model 2, after adjusting for the total number of devices per checkpoints, the total number of tests per checkpoints, and the average duration per checkpoint, a 10-unit increase in the number of all checkpoints per week was associated with a 12% decrease in the alcohol-involved crash rate for the following week (RR_lag1 = 0.88; 95% CI:0.80, 0.97). In Models 3 to 6, associations remained comparably weak but in the expected direction for short, long, small, and large checkpoints. In all models except Model 6 (large checkpoints only), associations were attenuated at a lag of 2 weeks compared to a lag of 1 week.

Checkpoints in the current week (lag 0) were associated with additional alcohol-involved crashes. For example, all checkpoints were associated with a 15% increase in the alcohol-involved crash rate for the current week (RR_lag0 = 1.15; 95% CI: 1.05, 1.25). In Model 2, an additional 10 devices per checkpoint per week was also associated with a 1% decrease in the alcohol-involved crash rate for the same week (RR_lag0 = 0.99; 95% CI: 0.99, 1.00). All covariates were not associated with the alcohol-involved crash rate, except for a quadratic association for time (i.e., for week and week-squared).

Theory and empirical studies provide strong evidence that sobriety checkpoints are causally related to fewer alcohol-involved motor vehicle crashes (Bergen et al., 2014; Elder et al., 2002; Erke et al., 2009; Shults et al., 2001). Research efforts are now required to identify the optimal configuration of individual checkpoints within an overall program to balance public health benefits while minimizing costs to law enforcement (Bergen et al., 2014). This analysis of sobriety checkpoints conducted by the Queensland Police Service identified that checkpoints are associated with fewer alcohol-involved crashes for approximately one week, and that there was no detectable difference in associations based on two key modifiable features of checkpoint sites—their duration and size. This research accords with previous studies of sobriety checkpoints and alcohol involved crashes. In addition to confirming the abundant published analyses demonstrating that checkpoints are generally associated with fewer alcohol-involved crashes (National Academies of Sciences & Medicine, 2018), this work also corroborates Stuster and Blower’s finding in six California communities that smaller checkpoints staffed by 3 to 5 officers had similar effect sizes compared to larger checkpoints staffed by 8 to 12 officers (Stuster & Blowers, 1995). Further, the temporal extent over which we observed associations in Brisbane is also consistent with our previous work in Los Angeles, California, where we identified individual sobriety checkpoints were associated with fewer subsequent alcohol-involved crashed over periods of approximately one week (Morrison et al., 2019). Prior studies (Morrison et al., 2019; Stuster & Blowers, 1995) detected no such relationships for saturation patrols (i.e. heightened police activity in a specified area without a dedicated checkpoint site; Fell, Lacey, & Voas, 2004). This collective evidence suggests that individual sobriety checkpoints are associated with fewer alcohol-involved crashes for approximately one week and that checkpoint duration and size do not affect these associations, but that having physical and observable checkpoint sites is an important element of the intervention design.

The findings of the current study are mostly consistent with general deterrence theory. According to this theory, the causal mechanism through which sobriety checkpoints affect drunk driving and alcohol-involved crashing is through increased perceived risks of detection and punishment for all drivers within a population at subsequent times, not just those who are caught offending while the checkpoint is in operation (Homel, 1993). Thus, the most important feature of a checkpoint is the number of people who are aware of its presence. Larger checkpoints with more officers testing more drivers will theoretically have the same impact as smaller checkpoints with fewer officers testing fewer drivers, provided the same number of motorists drive past or otherwise become aware of its presence. For that reason, longer checkpoints will theoretically have stronger effects than shorter checkpoints, although we did not detect such an association. It is possible that other checkpoint features not assessed here, such as accompanying advertising (e.g. marketing blitzes), are more important contributors to checkpoint effectiveness than the precise size and duration of each site (Saltz, Paschall, & O’Hara, 2021).

This work has important implications for law enforcement resources and public health. Police officers identify several important impediments to implementing sobriety checkpoint programs, including boredom, perceived risks for officers, perceived ineffectiveness, and economic constraints (Ross, 1993). Our results suggest it may be possible to reduce the human and financial costs associated with checkpoints by reducing checkpoint duration and size. Smaller, shorter checkpoints with fewer officers involved will reduce operational costs because they require fewer officer-hours.

As expected, we detected strong positive associations between counts of checkpoints and the incidence of alcohol-involved crashes during the same week (lag 0). This association underscores a key limitation of using an observational study design to relate checkpoints to crashes. Queensland Police Service has intimate local knowledge, decades of experience, and large volumes of driver impairment data to guide operational decisions about where and when to locate sobriety checkpoints. Systematically conducting checkpoints during weeks when drivers are more likely to be impaired is an important component of this preventive intervention (Community Preventive Services Task, 2014), but, from a research perspective, it also introduces endogeneity that manifests as a positive statistical association. Identifying the immediate deterrent effect of checkpoints on crashes during the same week requires an experimental design in which checkpoints are randomly distributed across time.

Other limitations may have affected these results. First, we did not have access to direct measures of checkpoint sites (day, time, duration, size, tests); rather we reverse engineered these measures using a hierarchical cluster analysis based on space-time data for individual breath tests. The identification of checkpoint sites appears to have criterion validity when compared to Queensland Police Service’s qualitative operations logs, but we were not able to assess validity quantitatively. Further, the number of unique latitude-longitude coordinates as a proxy measure for the number of officers present may be non-differentially misclassified and could inflate standard errors. Other misclassification could bias results in either direction. Additionally, the observational design critically limits our ability to assert that weeks are exchangeable units. We controlled statistically for Queensland Police Service differentially choosing to hold checkpoints during weeks when drunk driving and alcohol-involved crash incidence is greater, which will also account for systematic negative autocorrelation when high risk weeks are followed by low risk weeks. However, there may be other unmeasured confounders that affect these analyses and impede causal inference.

Comprehensive approaches to reducing alcohol-involved harms use a broad range of evidence-based strategies (Babor et al., 2010). Sobriety checkpoints are universal prevention strategy with a clear theoretical basis and strong empirical support as an intervention to reduce alcohol-involved crashes (Bergen et al., 2014; Elder et al., 2002; Erke et al., 2009; Shults et al., 2001). The intervention complements other evidence-based strategies, and should be part of a comprehensive approach to reduce the burden due to alcohol-involved crashes (National Academies of Sciences & Medicine, 2018). This analysis adds that individual checkpoints are associated with fewer alcohol-involved motor vehicle crashes for approximately one week, and that less resource intensive checkpoints of shorter duration and with fewer officers may achieve comparable public health benefits while reducing costs to law enforcement.