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Road Traffic Volumes Methodology

There are two essential data requirements for calculating the total vehicle-kilometres from odometer readings:

  • The number of vehicles in the fleet during the reference period
  • Regular odometer readings from roadworthiness tests

The number of vehicles in the fleet are obtained from the National Vehicle and Driver File (NVDF), administered by the Department of Transport. Odometer readings are available from two separate administrative sources; the National Car Testing (NCT) Service and the Commercial Vehicle Roadworthiness Testing Service. These sources are linked to the NVDF by vehicle registration number.

The principle unit of measurement used to assess Road Traffic Volumes is vehicle kilometres travelled (VKT). VKT is an aggregate estimate of the distance travelled by all vehicles in the fleet.

The distance travelled by a particular vehicle is evident from its odometer readings. Odometer readings are typically captured for private cars (four years old or more) at least every second year. Odometer readings are typically captured for commercial goods vehicles, public service vehicles and other selected categories of vehicles on an annual basis.

The difference between two successive odometer readings is the distance travelled by the vehicle between the road worthiness tests. This distance is then distributed across the intervening months. This distribution is not even. It is weighted according to a monthly traffic index created from traffic counter data (these traffic counter data are collected on National routes by Transport Infrastructure Ireland (TII)).

The monthly traffic index is particularly useful when the last available odometer reading for a particular vehicle predates the reference year. The index allows the historic rate of kilometre accrual of the vehicle to be extrapolated into the reference year in line with the contemporaneous traffic trend. A limitation of this method is that it is not possible to disaggregate the traffic index by vehicle type. Furthermore, differential traffic patterns caused by the Covid-19 pandemic may not be perfectly reflected in the 2020 and 2021 estimates.

Where odometer data is available, the fleet is divided into groups based on taxation class, fuel type, engine size, etc. For each group, the vehicle kilometres travelled by the sample vehicles is taken as representative of the group as a whole. The vehicle kilometres of all the groups are then aggregated to estimate the VKT of the fleet.

It should be noted that:

  • The number of vehicles in these estimates are based on continuous activity throughout the reference year. Thus, a vehicle active for just six months of the reference year has a population weighting of 0.5, whereas a vehicle active throughout has a weighting of 1.0.
  • Kilometres driven by Irish registered vehicles in other countries are included.
  • Kilometres driven by foreign registered vehicles on Irish roads are outside the scope of the survey.
  • Vehicle kilometre estimates and average kilometre estimates for motor cycles, tractors and machinery may be unreliable (minimal odometer data is available for these categories of vehicles).
  • Vehicle kilometre estimates and average kilometre estimates for private cars less than four years old are based on the extrapolated trends of older private cars (private cars less than 4 years are not subject to the NCT).
Preparation of the release

The NVDF, NCT and Commercial Vehicle Roadworthiness data files are obtained annually by the CSO. The CSO performs validations and consistency checks on the data and eliminates implausible and erroneous odometer readings.

History

The road traffic volumes tables were first published in 2008 as part of the annual Transport publication and cover a time series from 1999 to date.

In 2011, the methodology was updated to incorporate road traffic counter data from the NRA. The use of road traffic counter data allowed for a more nuanced estimation method, more likely to reflect full year-on-year changes. As an added advantage, seasonal driving behaviour could also be estimated.