asked Her Majesty's Government:

Further to the Written Answer by Lord Bassam of Brighton on 2 April (WA 185), whether the measurement of the carbon effect of a scheme being evaluated under the new approach to appraisal will be directly proportional to the aggregated time savings and distance travelled in the “with” and “without” interventions under consideration. [HL3064]

The carbon impact of an intervention appraised under the new approach to appraisal is directly proportional to the change in the fuel consumed. For highways, fuel consumption is related to speed. The relationship with speed is not linear. Fuel consumed per kilometre is high at low speeds, initially falls as speeds increase, but then increases again.

Thus, an intervention may improve speeds (and, therefore, provide time savings) but result in either an increase or a reduction in fuel consumption and carbon. Which outcome occurs will depend on where speeds with and without the intervention lie on the fuel consumption curve.