The Government has commissioned studies to assess the impact of vehicle speed on emissions, for example undertaking a high-resolution emission modelling approach to understand changes in vehicle speeds and how it relates to emissions. The Department uses the best available evidence to model emissions in urban areas, including evidence linking vehicle speeds with emissions based on international guidelines. The evidence is that the factors influencing emissions are complex and the relative impact of vehicle speed, acceleration/deceleration and distance travelled (which are all related to journey times), will vary depending on the local situation. In addition, the Government’s Joint Air Quality Unit helps local authorities to calculate NOx emissions in their local areas using outputs from traffic models, which use a range of parameters including speed and distance travelled (which influence journey time) and the number of vehicles. The results of these parameters are contained in their Full Business Cases which are all published documents.