At the strategic level, outputs from spatially coarse transport models are likely to give a fairly broad picture of the impact of options on CO2. Total CO2 emissions for road traffic can be calculated according to the method in DMRB 11.3.1. Care should be taken when interpreting the results of traffic models that do not account for intra-zonal trips or are coarse in urban areas. It may be appropriate to perform upper bound sensitivity test on model output.
Emissions from rail represent a very small proportion of the total transport emissions of CO2, about 3.6%. The data with regard to the emissions associated with the running of diesel and electric trains is at present limited. The research is ongoing to obtain more robust evidence to use for the estimation of emissions from rail. Prior to the outcome of this research the Transport Scotland recommends the use of the Rail Emission Model Final Report that was produced for the Strategic Rail Authority. This provides estimated emission factors and detailed data for individual diesel and electric train types. This is published on the Department for Transport website, and is available at:
http://www.dft.gov.uk/pgr/rail
The impact of a project on emissions should be calculated for each year over the 60 year appraisal period. It is accepted that extrapolation between model years may be required and this should be done in an appropriate manner.
