impacts: "In summary, whereas heavy goods vehicles speed limits in motorways are in line with the optimum speed in terms of energy and CO2 reductions per vehicle-km (80–90 km/h), decreasing car passenger speed limits in motorways could lead to substantial benefits." "On the other hand, energy and emissions benefits from more stringent speed limits on local roads (e.g. from 50 to 30 km/h) are less clear. The key argument for lower speeds on local roads is therefore the desirability of a safer and more tranquil local environment, rather than environmental considerations."