asked Her Majesty's Government:

What is their estimate of the effect on fuel consumption of raising average speeds of traffic by two per cent from the present level. [HL5346]

Department for Transport officials have not analysed the impact of raising average speeds of traffic by 2 per cent, and so do not have an estimate of the effect on fuel consumption.

The impact on fuel consumption of raising average speeds will primarily depend on whether raising the average speed on different types of road will move individual vehicles closer to or further away from their optimal fuel efficient speed.

The optimum speed for most cars is around 40 to 45mph, thus for roads where the average speed is below this optimal speed, an increase in the average speed may reduce fuel consumption for a given journey, although this may also have a negative impact on road safety and accidents. For roads where average speed is above this optimal speed, an increase in the average speed may increase fuel consumption for a given journey.