The Detailed Modelling of Vehicle Greenhouse Gas Emissions

The Detailed Modelling of Vehicle Greenhouse Gas Emissions


C Shaw, SIAS, UK


SIAS in collaboration with TRL on behalf of Transport Scotland has developed a vehicle emissions module for traffic microsimulation models based on the PHEM engine model. This session highlights the need for such a module and details its development.


The reliable estimation of emissions from transport vehicles is an increasingly important issue within the transport planning community as targets for local and global air quality continue to become more stringent. Traditional approaches for estimating vehicle emissions have relied on relatively crude methodologies that relate emissions to average vehicle speeds and therefore often include significant error by over or under-estimating the emissions caused in stop/start conditions.

As technology has improved, road traffic simulation software has been developed that enables a more detailed representation of traffic flow down to the individual vehicle level. The more detailed outputs from microsimulation models in turn enable more detailed emission estimation methods to be adopted that relate individual vehicle speeds and acceleration rates to tailpipe emissions at discrete intervals along each trip.

On behalf of Transport Scotland, SIAS in collaboration with the Transport Research Laboratory has developed a state of the art vehicle emissions module. This takes outputs from microsimulation models for individual vehicles at each simulated timestep, typically twice a second, and estimates the tailpipe emissions based on their current speed and acceleration. The module uses detailed Instantaneous Emissions Modelling factors derived by TRL using the Passenger car and Heavy Duty Emissions Model (PHEM) developed by the Technical University of Graz.

Testing of the module has demonstrated significant differences between the emissions levels estimated by traditional methods and the more detailed PHEM based method, particularly in congested urban areas where stop-start conditions prevail. This could be critical when assessing the impacts and benefits of transport scheme and policy interventions.


Association for European Transport