A6 RUSHDEN – HIGHAM FERRERS BYPASS ... - Highways Agency

4.11 This re-forecast benefit is relative to the same 1998 base year as used in COBA.However, it is necessary to convert it to 2002 values, to permit comparison withresults from other schemes evaluated under POPE.4.12 To achieve this conversion, the predicted benefits from COBA are ‘undiscounted’,separately for each year. The undiscounted benefits are converted to 2002 pricesusing the RPI, and then re-discounted to 2002 at 3.5% p.a.. The separate annualbenefits are then summed. The resulting predicted benefit at 2002 base is found tobe 23% higher than at 1998 base. Therefore the ‘observed’ 1998 benefit isincreased by this amount to derive a corresponding 2002 value.4.13 Monetised accident benefits are treated in exactly the same way, with the ratio ofobserved: predicted accident savings being used to adjust the predicted 30-yearaccident benefit.Predicted BenefitsRe-evaluated Time Benefit4.14 The COBA prediction was that 79,981 vehicle-hours would be saved in 2008, onthe key links where observed information is available, whereas the actual saving is133,287 vehicle-hours. This is 66% higher than the predicted value. The reason forthis is that although the actual time savings per vehicle are a little lower thanpredicted, there are many more vehicles on the network than predicted, which arebenefiting from the savings. In particular, traffic flows were under-predicted in thesouthern part of the network.4.15 The 30-year monetary benefit corresponding to the predicted time saving was£17.6 million. Scaling this up by 66% gives an ‘actual’ monetary benefit of £29.3million.Table 4.2 – Time Saving and Monetary BenefitVehicle Hourssaved in 2008Benefit over 30 yrs (£m)1998 Base 2002 BaseCOBA 79,981 17.6 21.7Actual 133,287 29.3 36.34.16 Converting this benefit from a 1998 present-value year to 2002, using theprocedure described above, results in a value of £36.3 million. The time benefit isshown in Table 4.2.Re-evaluated Accident Benefit4.17 The COBA prediction was that 3.2 accidents would be saved per year, whereasthe actual saving is 1.8 accidents, which is 56% of the predicted value. The 30-year monetary benefit corresponding to the predicted time saving was £4.9 million.Scaling this down by 56% gives an ‘actual’ monetary benefit of £2.8 million.42