This section gives an overview of road traffic volumes.
The vehicle-kilometres estimates are based on odometer readings. There are two essential data requirements for calculating the total vehicle-kilometres from odometer readings:
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)).
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