Road motor vehicle emissions

  • Image, Road motor vehicle emissions.

    2018 air indicators are now available

    Road motor vehicles emit a range of air pollutants from their exhausts, and from brake and tyre wear. They are the main human-made source of nitrogen oxides and carbon monoxide emissions. Exposure to these pollutants can damage health, with effects ranging from respiratory irritation to cancer.

    We classified Road motor vehicle emissions as a national indicator.

    Key findings

     Decreasing trend (improving state)

    Estimated emissions of key pollutants from road motor vehicles decreased 26–52 percent between 2001 and 2013.

    • Estimated annual emissions decreased for:
      • particulate matter 10 micrometres or less in diameter (PM10), 26 percent
      • particulate matter 2.5 micrometres or less in diameter (PM2.5), 28 percent
      • nitrogen oxides (NOx), 37 percent
      • volatile organic compounds, 52 percent
      • carbon monoxide (CO), 46 percent.
    • These decreases occurred despite an increase in road motor vehicle use (vehicle kilometres travelled) of 12 percent. Improvements in our road motor vehicle fleet and the quality of our fuel likely caused these decreases.

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    Definition and methodology

    Road motor vehicles range from passenger vehicles to heavy commercial vehicles, including petrol and diesel vehicles. Farm and construction vehicles are not included. While road motor vehicle travel predominantly involves petrol vehicles (approximately 73 percent of vehicle kilometres travelled), diesel vehicles (approximately 27 percent of vehicle kilometres travelled) contribute the majority of air pollutants from road motor vehicles – specifically particulate matter and nitrogen oxides (NIWA, 2015; Parliamentary Commissioner for the Environment, 2015).

    Our road motor vehicle emissions are estimated using the Vehicle Emissions Prediction Model (VEPM). The model considers kilometres travelled for different vehicle types, the estimated emission factor for each vehicle type, and the corresponding average vehicle travel speed. The emissions estimates include vehicle exhaust and brake and tyre wear.

    Trend assessments are conducted on annual estimates and may not fully account for seasonal effects. The assessments do not indicate any underlying drivers behind the decreasing trend. However, improvements in our motor vehicle fleet and fuel quality probably account for the decrease.

    The VEPM provides best estimates and generally corresponds well to short-term studies measuring vehicle emissions at selected road sites. However, while the VEPM shows a decrease in NOx (the collective term for nitrogen dioxide (NO2) and nitric oxide (NO)) emissions, some monitoring shows no corresponding decrease in NO2. Possible reasons for this include:

    • limitations in the model 
    • a result of the total NOx emissions decreasing while the NO2 component increases – this occurs in some newer vehicles, particularly diesels, which have a much greater proportion of nitrogen dioxide than petrol vehicles
    • emissions from real-world situations (where vehicles are driven in a variety of conditions) are often much higher than those found in emissions approval tests.

    Data quality

    We classified Road motor vehicle emissions as a national indicator.


     This national indicator is a direct measure of the ‘Emissions of air pollutants from transport’ topic. 


     The accuracy of the data source is of high quality.

    See Data quality information for more detail.


    NIWA (2015). Uncertainty estimates for the national PM10 indicator. Including an update of vehicle emissions estimates to include 2013. Wellington: Ministry for the Environment.

    Parliamentary Commissioner for the Environment (2015). The state of air quality in New Zealand: Commentary by the Parliamentary Commissioner for the Environment on the '2014 Air domain report'. Available from


    Published 21 October 2015

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