Evaluating the impact of lowering blood alcohol concentration limits for drivers on

Background Drink-driving is an important risk factor for road traffic accidents (RTAs) which cause high levels of morbidity and mortality globally. Lowering the permitted blood alcohol concentration (BAC) for drivers is a common public health intervention enacted in countries and jurisdictions across the world. In Scotland, on 5th December 2014, the BAC limit for drivers was reduced from 0·08 g/dL to 0·05 g/dL. This study evaluated the impact of this change on RTA and alcohol consumption outcomes. Methods We employed an observational, comparative interrupted time series design using data from the whole of the intervention (Scotland) and control (England & Wales) groups for the period January 2013 to December 2016. Weekly counts of RTAs were obtained from police accident records and weekly off-trade and four-weekly on-trade alcohol consumption was estimated from market research data. We used data from automated traffic counters as denominators to calculate RTA rates. We estimated the effect of the intervention on RTA and alcohol consumption outcomes using negative binomial panel regression and seasonal autoregressive integrated moving average models, respectively. Findings The change in drink-drive legislation was associated with a 1% increase in total RTA rates in Scotland (rate ratio (RR) 1·01; 95% CI 0·94, 1·08; p=0·77) after adjustment for seasonality and underlying temporal trend. When expressed relative to England & Wales, where the intervention did not happen, the association was estimated as a 7% increase in total RTA rates (RR 1·07; 95% CI 0·98, 1·17; p=0·10). Similar findings were observed for serious/fatal RTAs and single vehicle night-time RTAs. For alcohol consumption, in Scotland, the change in legislation was associated with a 0·3% decrease (relative change) for consumption measured by per capita off-trade sales (-0·3%;-1·7%, 1·1%; p=0·71) and a 0·7% decrease in per capita on-trade sales (-0·7%;-0·8%, -0·5%; p<0.001). Interpretation Lowering the BAC limit to 0·05 g/dL from 0·08 g/dL in Scotland was not associated with a reduction in RTAs, but was associated with a small reduction in per capita on-trade alcohol sales. One plausible explanation is that the legislative change was not suitably enforced, for example with random breath testing measures. Our findings suggest that changing the legal BAC limit in isolation does not improve RTA outcomes. These findings have significant policy implications internationally as several countries and jurisdictions consider a similar reduction in BAC limit. Funding This project was funded by the NIHR Public Health Research Programme (project number PHR 14/186/58). Registration ISRCTN registry (ISRCTN38602189), date applied 02/05/17, date assigned 27/06/17 https://doi.org/10.1186/ISRCTN38602189