Post implementation scheme evaluations : were the COBA assessments accurate ?

Eleven post implementation studies, of relatively simple recently opened trunk road schemes, have now been completed. The objectives of the studies were to quantify outturn scheme costs and benefits, using post opening data, and to identify the reasons for any differences between forecast and outturn NPV. An incremental approach was adopted whereby the original order publication stage COBA assessments were updated to be consistent with the current COBA economic parameters and then re-evaluated using newly collected data on traffic volumes, accidents and journey times. The paper discusses the main causes of the differences between the forecast and outturn NPV and also highlights areas in which current scheme appraisal procedures could be improved.