Finding the denominator

By Eric Crampton 19/12/2012

We simply cannot tell whether changing the drink driving limit to 0.05 from 0.08 is a good idea unless we have some idea what proportion of drivers on the road at different times of day are in that range. We can talk a lot about the numerator – how many people involved in accidents have had something to drink. But without the denominator, we cannot make sense of the numerator. 20 accidents could be a huge or a tiny proportion of drivers on the road who have similar blood alcohol readings.

Most likely, drivers in the 0.05 to 0.08 range would be over-represented in the accident stats after correcting for time-of-day and day-of-week effects. But that doesn’t tell us whether reducing the drink driving limit would be a good idea. Rather, we have to weigh up the likely reduction in consumption in that range given a change in the law, and the likely consequent reductions in accidents in that range, against forgone consumer surplus among those who would have otherwise been driving in the 0.05-0.08 range without adverse incident. I don’t know which way this would turn out, which always makes things more fun.

The New Zealand Police told me that it’s impossible to have any automated gathering of that data – their machines are not set up to keep those running tallies. And it didn’t look like anybody else was collecting the data. So I tagged along with an alcohol checkstop unit on Saturday night to scope out what would be needed for a survey – how many research assistants would need to tag along to have a decent chance of getting accurate data while still staying out of the way. Over about an hour on Victoria Street, 173 drivers returned no alcohol; 13 were in the 0-250 mcg range (under 0.05); 2 were in the 250-400 range (0.05-0.08); 2 were sent on for evidential readings as they seemed to be above 400 mcg. The officers noted it seemed to be a pretty quiet night – traffic was light and few drivers were in the 250-400 range as compared to other nights. And, surprisingly, one of the officers reported that she was sure that someone else had been collecting data of this sort.

I subsequently heard back from the right person at the Ministry of Transport that they have run a short survey assessing the proportion of drivers on the road in the various BAC ranges, or at least tallying the numbers registering 0, 0-250, and 250-400. And, even better, they’ve promised me the raw data mid-January. It’s limited in that they were only looking at Friday and Saturday nights, but so long as I can sort out the proportion of accidents that happen at the same times, I can deal with it.

I’m very glad I’m not going to have to re-create the wheel on this one. The data collection looked like it wasn’t going to be the easiest thing, and I was starting to have nightmares about just what the University administration paperwork was going to be needed to get permission to send RAs out with the Police, even were I to have found external funding.

Many thanks to the Officers who let me tag along for the night, the Canterbury Road Policing Manager, and Mike McCosker for getting me in touch with the right people to get things going. I think it’s going to be pretty useful knowing how this data gets generated when I go to play with it in the early new year.