Mark Kleiman and I have written extensively about intervention programs for substance-involved criminal offenders which combine regular drug/alcohol testing with swift, certain and mild consequences for use and non-use (see for example here and here).
One of the prototypes is 24/7 Sobriety, a South Dakota program that sentences repeat drink drivers to twice a day breathalyzation. The chart below uses the most recent National Highway Traffic Safety Administration data to show how the safety of the state’s roads has improved since the program began.
I used to display these sort of data in the conventional metric of alcohol-involved auto-fatalities per million or hundred million vehicle miles traveled. That didn’t work well because neither policymakers nor the general public had a sense of what it really meant. Few people have a firm concept of how much driving a million vehicle miles is, or how many million miles Americans collectively drive per year.
The below chart adopts a different approach, namely expressing how dangerous it is to drive in South Dakota relative to the US as a whole. In the two years prior to the founding of 24/7, the risk of dying in an alcohol-involved auto accident per mile driven was almost twice as high in South Dakota as in the US as a whole. In the next two years, during which 24/7 began steadily expanding across the state, the risk dropped to about 1.7 times as great as the US as whole. Since the program went statewide in 2007, South Dakota is approaching being as safe as the national average.
Beyond ease of comprehension, another advantage of this way of breaking down the data is that it adjusts for the fact that the US as a whole has seen a drop in drink driving fatalities. Because the ratio between the state and national risk displayed in the chart is dropping, we can conclude that the welcome change in South Dakota is beating the secular national trend substantially rather than just being a local case of a broader trend.
Population-level relationships like this may not be replicated in individual data. Dr. Beau Kilmer of RAND Corporation is now doing more rigorous work on connecting individuals’ outcomes in 24/7 to traffic safety and crime impacts, and he will have a study of those issues out in a journal next week.
Note also that 24/7 sobriety is not the only effort South Dakota mounted against drink driving, so it could be that other policies (e.g. sobriety checkpoints) are causing the change evident in the chart. However, Montana, a similar nearby state with a serious drink driving problem tried all those policies as well and stayed extremely unsafe until 2010. In that year they began their own 24/7 sobriety program, and experienced a 40% decrease in the rate of alcohol-involved auto fatalities per mile driven. Most encouraging.