I have previously written about the European Directive for promotion of Clean and Energy efficient vehicles.
When comparing the environmental impact from different transports, second to bikes, buses nowadays have the lowest impact, lower than the trains running on fossile and emission free electricity.
I have used the methodology to calculate how the life cycle impact of a bus has evolved during the years. The fuel consumption and the emissions have decreased over the years.
When looking at the environmental load per passanger kilometer I used 20% occupancy for the bus and 1.5 persons for the car. For the train I used data from the ADEME study I previously given reference to and assumed an energy efficiency improvement historically of 0.5% per year.
The bus is a moving target. And, it is expected to continue to improve in the future. Hybrid buses offers the lowest impact today