Crushing and crashing

The Herald says “Police Minister Judith Collins has released figures to show crushing boy racers’ cars has worked“.  The data are more consistent with the political interpretation than is usual for claims about crashes, but not as strong as the Minister would like us to think.

Here’s a graph of the data (supplied to the Herald by the Minister’s office), showing crashes, injuries, and deaths where the police reported ‘racing’ as a cause:

It’s fairly clear that something changed. Based purely on the graph you’d say the downwards trend started after 2007; 2009 isn’t unreasonable, but it fits the data a bit less well. This an example of a graph being much more useful than a table.

The next thing to check is other crashes — the road toll has been down in recent years, so this could just have been a general improvement. It’s not; the evidence for a change is a little weaker when considering racing deaths or injuries as a proportion of all fatal or injury crashes, but it’s still there.

In principle there could have been changes in reporting, but it’s hard to see how a government crackdown would make police less likely to report ‘racing’ involvement in a crash.

Finally, there’s publication bias.  The reporter, Nicholas Jones, didn’t notice that Ms Collins was back and decide to pull figures on car crushing; the Minister decided to release the figures. She wouldn’t have done that if they didn’t look favourable. It’s hard to tell how much to discount the evidence for that, but a discount is needed.

Overall, the data are definitely consistent with a deterrent effect of car crushing, but the evidence isn’t all that strong — the best fit to the data suggests things changed earlier than 2009, and looking at the numbers was the Minister’s idea, not the reporter’s.

Updates:

• in addition to the useful comments, I’ve been pointed to Dog & Lemon where Clive Matthew-Wilson says there is reason to believe the ‘boy racer’ thing was already going away on its own.  If so, that would fit the trend starting earlier than the legislation.
• If you check the crash numbers against the Road Crash Statistics system they don’t match.  I think that’s because Table 26 of the Road Crash Statistics only includes crashes causing injury or death — that’s explicit in the 2012 spreadsheet, and I think it’s still true.