The Department for Transport publishes estimates of road traffic and CO2 emissions from road transport in “Transport Statistics Great Britain”. The latest edition can be found at:
This data can be used to provide an estimate of the average level of carbon dioxide per mile for road traffic. For 2007, the latest year for which data are available, this figure is 379gCO2/mile.
“Transport Statistics Great Britain” does include similar data for road traffic and CO2 emissions for other EU-27 and some G8 countries, but the road traffic and CO2 emissions data are not held for the same year and so a similar calculation of average CO2 per mile can not be made.