There is a handy infographic on Council’s website. Unfortunately, details of how the benefits have been calculated are not publicly available.

What we do know is that Council intends to spend $190 million to build the bridge. Council believes trips per day will rise from 5,300 in 2021 to 6,100 in 2036.

Making a couple of assumptions, we can work out the level of benefit per trip required for this to cover the capital costs. First we assume a 25 year evaluation period (2020-2044) and a 7% real discount rate. We also assume the asset has a 50 year life and include an offset residual value on the capital cost. The capital cost attributable to the evaluation period is a present value of $172.5 million.

Then we extrapolate the average crossings, which rise on an annualised basis from 1.9 million trips in 2021 to 2.4 million trips in 2044. This implies some 54 million trips will take place (2021-2044). However, using trips as a stand in for benefits, a trip today has a stronger present value than a trip tomorrow. The present value of all the trips had they occurred today is 24.5 million.

The capital cost per trip is $3.18 undercounted. In discounted terms, this is $7.03.

Given ratepayers are paying for it, one would hope Council is confident benefits are at least $7.03 per trip in benefits.