the procedure of cost benefit analysis(cba) is summaized in what follows.
1)benefit and cost in the service starting year should be calculated with the methods described in "3.2 benefit measurement" and "3.3 cost accounting". then benefit and cost in other year should be estimated based on the traffic volume which is extrapolated from service starting year. They are discounted to the present value in the base year with the social discount rate 4%
2)three indices for economics evolution( cost benefit ratio, economic, Net Present Value and economic internal rate of return) are calculated with the aboved obtained values.
3) estimates of key elements in the CBA like future traffic volume,project costs and so on,often turn out to differ from the realized ones because of changes in the economic and environment and in related projects. It is inevitable that is index for economic Evolution derived from these estimates should have uncertainty. Sensitivity analysis should be therefore performed by varying the values of future traffic volume, construction costs and so on, so that their influence on each index for economic evaluation can be verified