Better than nothing

Our helpful commenters provided alternative suggestions on how the intersection car crash rates could have been standardised, instead of using population of each region

• Luis Apiolaza suggested number of registered vehicle, available from NZTA
• David Welch suggested person-km travelled, also available from NZTA

Number of registered vehicles is a bit of a pain, because it is reported for postal districts, not for regions.  I assumed that postal districts are a partition of regions (I couldn’t confirm or deny this immediately), and did a bit of Wikipedia. Presumably an NZ journalist could do this quicker than me, and would already know, for example, that the Canterbury:Otago border passes between Timaru and Oamaru.

It doesn’t matter a lot which standardisation you use.  The graph below (click to embiggen) shows all three, scaled so NZ as a whole is 100.  The orange bars are by population, the brown bars by km, and the maroon bars by registered vehicles.  The most striking difference is probably for Wellington, where the rate per registered vehicle is high: there are fewer registered vehicles per capita than in the rest of the country.

All three versions confirm that there is much less variation than the Herald story would suggest, and that  urbanisation is likely responsible.  Error bars would be nice, but I don’t know what the uncertainties in the NZTA denominators are like.