The two VKTs

On Monday I wrote about the AAs annual congestion report. One aspect I commented on was the difference in the vehicle kilometres travelled (VKT) numbers between the ones I got from the NZTA earlier this year and what the AA have presented.

I decided to look into it more and I found there are not one but two different regional VKT measures, one from the NZTA and one from the Ministry of Transport. Helpfully a paper recently uploaded by the MoT breaks down the differences between but also reveals that “each of which have their limitations, and which are not always consistent with each other“.

So what are the two ways of calculating VKT.

MoT version

The MoT use odometer readings taken when vehicles are inspected for a Warrant of Fitness and are aggregated based on the area the vehicle was assessed in. In other words, if you take your vehicle for a WoF in Auckland, all of the VKT you’ve driven will be assigned to Auckland. This sounds good on the surface but does have some flaws.

For the odo-data, a major potential source of error is that the VKT data by region has to be inferred based on the region where the vehicle was inspected. Of course, vehicles can actually be driven in any region, and many vehicles incur a substantial number of kilometres in a region other than the one where they were inspected. If these miss-assigned VKTs were completely random, they would cancel out and there would be no problem. However, some regions have lots of tourists driving in from other regions (such as the West Coast) or travelling across them between other regions (Waikato, Manawatu-Wanganui). Also, some regions may be the base for big fleets of rental cars or trucks that actually get driven a lot in other regions (Auckland). A less significant source of errors are timing issues: it would be nice if all odometers got read on the same date each year, but alas, they aren’t, so some adjustment has to be made to assign the VKTs to the right year

I do wonder what the changes to the WoF rules in 2014 may have had here. In particular, new vehicles don’t need a new warrant of fitness for their first three years and there have been a lot of new vehicles sold in the last few years.

NZTA version

The NZTA use data from the Road Assessment and Maintenance Management (RAMM) database which tracks the usage of all public roads based on traffic counts. This is how the NZTA are also able to break down usage by state highways and local roads and other splits. But using traffic counts has its own problems.

Production of the RAMM data is much less straightforward. Some kind of model must be used to multiply the traffic counts on each road by an assumed distance travelled, then this data on each road must be aggregated. Clearly, this process requires a number of assumptions on the part of the modeller, which may or may not be correct. In the case of at least one region (Auckland), an even higher level modelling technique is used, based on a sample of carriageway counts from around the region. Also, the counts are not done on the same day each year, and may not even be done each year at all, so, as with the odo-data, there are timing issues. In particular, there is a possibility of significant lags between when traffic actually changes and when the changes get reported in RAMM.

Local roads are often only monitored infrequently and many small roads like cul-de-sacs don’t appear to be monitored at all, so it wouldn’t surprise me if some of the numbers were understated.

How do they compare

Given it is based on actual kilometres travelled from odometer readings, the MoT version will be fairly accurate at the national level. The graph below highlights the differences between the two measures with both showing an uptick in recent years but the MoT version being almost 1.5 billion kilometres higher than the NZTAs estimates for 2018.

But the difference in recent years has much more pronounced in Auckland. Given the explanation above I wonder if this has a lot to do with the growth in the economy and tourism resulting in a lot more travel outside of the region for fleets based in Auckland. The real number is likely somewhere between these two lines.

These results also have some interesting implications for how we measure travel per person given Auckland’s strong population growth over the last five or so years. The NZTA version shows a significant reduction in travel per capita, which could reflect a combination of factors, such as more people living in the city and not needing to drive, as well as our improved uptake of PT and active modes. With the MoT version it shows things at about the same level as they always were with a dip in the early 2010s likely related to the economy at the time.

What both the Auckland graphs above also show is that MoT numbers have flattened/dropped which is notable as the AA report on Monday claimed that the numbers were going up significantly with a 310km increase per capita and a total VKT increase of 900 million km to 15.5 billion. However, even using the MoT figures they show usage per captia use falling last year while the total measure is flattening – and peaked well under the 15.5 billion km the AA claim.

After seeing there are two different VKT measures it does worry me that people could cherry pick the one that suits their narrative best – it certainly wasn’t my intention when writing the post in May. Perhaps both agencies need to sit down together and come up with an agreed way of calculating regional VKT.

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