Assumptions and Limitations in Road Traffic Emissions Modelling

12 Sep 2022

Environmental Planning

Air quality

Limiting and, where possible, reducing road traffic emissions is an important part of improving local air quality, particularly in urban areas. On the city scale, keeping main pollutants', such as NO2 and particulate matter (PM10 and PM2.5), concentrations in line with recommended Air Quality Standards is important in not causing detrimental impacts on human health, particularly for those in urban areas and those who are more susceptible to changes in air quality. Management and mitigation of road traffic emissions plays a significant role in aiding the goal of limiting global greenhouse gas emissions. There are a number of suggested ways to mitigate and reduce road traffic emissions, including a transition to low/zero emission vehicles and promotion of more sustainable transport methods.

Air Quality Consultant Harry Porter explores whether it is beneficial for Air Quality Assessments to be more representative of real-world conditions but with reduced robustness, or to continue to follow the more conservative approach.

It is considered good practice for Air Quality Consultants to use conservative assumptions, when undertaking dispersion modelling of road traffic emissions as part of an Air Quality Assessment, in order to create an overall robust assessment. This approach is advantageous to the Client, and the environment, as it provides reassurance that a proposed development will not have a significant effect on existing or proposed sensitive receptors, with regard to air quality. 

Such assumptions are an important part of the Air Quality Assessment process, as we seek to demonstrate how development scenarios may affect the real-world environment. Assumptions include the identification of the most appropriate model input parameters and the appropriate application of tools used in the processing of results.

Ultimately, if predicted development impacts at sensitive receptor locations are within acceptable limits, as defined by Environmental Protection UK (EPUK) and the Institute of Air Quality Management (IAQM), despite the conservative assumptions, the resultant confidence in the acceptability of the proposed development is high. 

In order to achieve this robust scenario a number of assumptions are made when building the road traffic emissions models, including:

  • Due to the uncertainty over nitrogen dioxide (NO2) concentrations within the UK, which have not resulted in the previously expected reduction in roadside levels following the implementation of new vehicle emission standards, it has become common practice to utilise verification year emissions factors and background concentrations over the Site opening year for development scenarios. This assumption has been made even more robust, as Department for Environment, Food and Rural Affairs (Defra) recommends the continued use of 2019 as a verification year to avoid any further uncertainty and underestimation of pollutant concentrations, as a result of COVID-19 Travel Restrictions on 2020 and 2021 road traffic and emissions data in the UK. 
  • Other uncertainties within the model, such as the accuracy of meteorological parameters, including the surface roughness length and the minimum Monin-Obukhov length, plus the geometry of the modelled roads, further add to the task of estimating acceptable but robust modelled pollutant concentrations. 
  • The traffic data provided and used in the model should reflect, as accurately as possible, the real-world traffic flows along the road links used in the model. This includes annual average daily traffic (AADT) flows, percentage of heavy duty vehicles (HDVs) and, where possible, the average speeds along the modelled road links. Where assumptions are made, however, these tend to be conservative and traffic flows are often based on the maximum proposed quantity/scale of development (e.g. the greatest number of dwellings proposed for the site).
  • In general, Transport Consultants are primarily focused on vehicle numbers or passenger car units (PCUs), rather than the types of vehicles generated. As such, it is often assumed that the vehicle composition will remain static for future years, when in reality, the take-up of electric and other low-emission vehicles is rapidly increasing, with localised emissions reducing accordingly.


Challenges with a Robust Assessment Approach

The challenge with the assumptions and limitations in determining the model input parameters is to achieve the correct balance between presenting a set of robust,yet realistic, assessment scenarios that demonstrate both compliance and assertiveness of the developments’ potential impacts and their effects on the receiving environment.

In creating a robust assessment that utilises assumptions as part of the methodology, there is the potential for the model to predict pollutant concentrations that indicate a ‘moderate’ or ‘substantial’ impact, which would have a corresponding ‘significant’ effect at representative sensitive receptor locations. This would be due to the use of excessively conservative assumptions, with regard to emissions factors, background concentrations and traffic data, rather than the utilisation of the most realistic datasets for the development opening year. 


Case Study: Assumptions and Limitations in Road Traffic Emissions Modelling

Challenges with the robust assessment approach were observed in a real-world example where a combination of factors led to the prediction of ‘minor’ to ‘major’ significant adverse effects and new exceedances of the annual mean Air Quality Objective (AQO) for NO2, within an existing Air Quality Management Area (AQMA), as a result of a proposed development. 

Following the submission of the Air Quality Assessment, data from a new strategic transport model (STM), developed for the purposes of the Local Authority’s emerging Local Plan, became available. With the modelled pollutant concentrations in future scenarios being unusually high compared to the Council’s monitoring data within the assessment extents, it became evident that a review of the model inputs and parameters was required. This would ensure that the assumptions used had been reasonable and therefore, the results of the assessment were valid and reflective of realistic conditions. 

It was observed that the STM traffic flows for the verification year were significantly higher than those used for the Air Quality Assessment’s 2019 model verification scenario. Furthermore, the increase in flows from the verification year to the development opening year used in the Air Quality Assessment were viewed as unrealistically large, with a 25% increase in baseline traffic flows observed between 2019 (verification year) and 2026 (development opening year), on some road links. 

A comparison of the traffic flows, which were derived from earlier traffic models, and the new STM data, particularly on the roads where significant adverse effects and new exceedances of the AQO were predicted, was therefore undertaken. 

It was concluded that, the increase in baseline flows from the 2019 verification year to the development year (2026) was having the effect of excessively increasing baseline pollutant concentrations within the model, in conjunction with the conservative emission factors utilised in the assessment. This increase in traffic flows was found to result from a disconnect between the method used to derive the 2019 and 2026 traffic flows respectively, as these flows had originally been derived from two different transport models. Accordingly, the relationship between verification and future baseline year flows (2019 to 2026) was shown to have a large influence on the resulting significance of effects, in addition to the increase in flows resulting from the development itself. 

The use of the lower base traffic flows had the effect of reducing the model-predicted concentrations of nitrogen oxides (NOx) at monitoring locations within the assessment extents and therefore, over-inflating the adjustment factor calculated in the model verification process. Subsequently, this disproportionately increased the absolute concentrations for all of the future scenarios, and the magnitude of impacts that were detailed in the assessment. Accordingly, it was considered appropriate to update the model verification using the new STM data to better represent real-world traffic conditions within the assessment extents and to, therefore, align with the Local Authority’s monitoring data within the vicinity of the site.

Subsequently, in close liaison with the Transport Consultant and the Local Authority, it was agreed that an updated model should be developed for all model scenarios using updated traffic data and more realistic model input parameters, compared to the excessively conservative ones used previous, as detailed below:

  • Following submission of the original assessment, amendments and improvements were made to the STM from which the traffic data for the Air Quality Assessment were derived. This led to a better correlation between the verification and future baseline year scenarios, which allowed the assessment to focus on the actual impact of the development rather than apparent effects associated with disparity between the baseline traffic scenarios. Accordingly, updated verification and development year baseline traffic flows were used in the updated assessment. 
  • The modelled scenarios utilised emission factors that reflected a more realistic assessment approach and as such, development year (2026) emission factors were used for the assumed opening year scenarios. 
  • To further aid in the representation of a more realistic modelling scenario, development year background pollutant concentrations for the assumed opening year were utilised in the updated assessment. 
  • Additional updates to the model input parameters included speed corrections following a review of major junctions and reconsideration of electric vehicle uptake within the future scenarios’ emissions profiles. 



Following the implementation of the above changes to the road traffic emissions model, the pollutant concentrations at sensitive receptor locations no longer indicated any potential for significant effects or exceedances of the AQO as a result of the proposed development. This was in part due to the updated verification year traffic data leading to a reduction in the calculated adjustment factor, resulting in the model being more representative of local pollutant monitoring, and the more realistic approach of using the assumed opening year (2026) background concentrations and emissions factors for the development scenarios.

The case study above highlights the requirement to question whether it is beneficial for Air Quality Assessments to be more representative of real-world conditions but with reduced robustness, or to continue to follow the most conservative approach at all times, with the risk of over-predicting impacts on the air quality of the local area. A sustainable compromise may be to employ the use of professional judgement as well as to carry out the assessment jointly between the Project Team and key regulatory stakeholders, to make sure that an assessment remains realistic and continues to follow accepted EPUK/IAQM modelling guidance, where appropriate.


About Delta-Simons

Delta-Simons is a multi-disciplinary environmental and health and safety consultancy providing trusted advice and solutions to ‘Protect People and Planet’ through facilitating sustainable development.

We work with clients from multiple sectors including; industrial, commercial & residential developers, retailers, industrial manufacturers, fund managers, institutional investors, hi-tech companies, the public sector and charitable organisations. For the past 30 years, our team has provided a holistic suite of commercially astute environmental services and advice, designed to mitigate risk to both people and the environment, whilst removing the pain from planning or maintaining buildings and/or assets.

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About The Author

Air Quality Consultant Harry Porter joined Delta-Simons in October 2020 having completed an MSc degree in Environmental Science and Management at the University of York in 2018 with 4 years of experience in the air quality field as an Air Quality Consultant. Having gained experience in many different aspects of air quality, Harry's main areas of expertise include undertaking detailed air dispersion modelling assessments of road vehicle and commercial emissions and preparing factual and interpretive Air Quality Assessment  reports and Air Quality Environmental Statement chapters.  

An Associate Member of both Institution of Environmental Sciences (AMIEnvSc) and the Institute of Air Quality Management, Harry has ample experience in delivering essential, trusted environmental services to our clients across a range of sectors.

For further information about our Environmental Planning and Air Quality services, contact Harry:


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