Sustainable development, now an integral part of public policy, requires cultural changes that encourage "transversal", systemic and participative approaches. Implementing all this is, however, proving difficult. The use of "clean" energy is one solution: indeed, such energy sources are starting to become established in both the private and public transport markets. For instance, the specific nature of public transport routes makes it possible to use "clean" energy sources that have an impact on greenhouse-gas emissions: given that public transport vehicles always follow the same routes, they can be powered electrically via overhead lines (as is the case for trolleybuses and trams) or even via the ground in some cases (e.g. certain sections of certain tramways). Engines running on "greener" fuels have existed for a number of years now – for example, buses that run on biodiesel, colza oil, natural gas or LPG. Soon, there will even be buses that run on hydrogen (e.g. in Dunkirk and Toulouse)*. It will be possible to recharge hybrid engines more easily, with energy recovered during braking at bus stops, enabling the vehicle to then move off with reduced energy consumption. More generally, efforts are being made by manufacturers to reduce the weight of vehicles and optimise their dimensions.