An economic benefit of $68.61 million over the next 30 years could be achieved if the board of the New Zealand Land Transport Agency goes ahead with a $39 million upgrade of State Highway 1 through Caversham.
So far, the board has committed only to spending $6.6 million on design work and land purchase, but is considering various options to improve the highway - the most expensive costing more than $50 million.
The economics of three options were contained in a July draft of the ‘‘interim scheme assessment report'' to the board, obtained by the Otago Daily Times.
The report says by spending $12.69 million on the cheapest option, widening the two-lane section of the Caversham bypass to four lanes, an economic benefit of $39.68 million could be achieved.
By spending $39.75 million on the more extensive option 3b, which includes major improvements to the Barnes Dr intersection and Caversham Valley Rd to Lookout Point, a benefit of $68.61 million could be achieved.
No figures are given for the $50 million option, which includes a traffic bridge over the highway at Lookout Point.
The report sets out the benefits under four headings: travel time, vehicle operating, congestion costs and accident costs.
Most of the benefits would come from faster travel times and reduced accident costs (see boxes) and, according to a note to the board from transport consultants Opus, the boosting to four lanes of the two-lane Caversham bypass would provide the greatest benefit.
The note describes the bypass as a ‘‘throat'' for city-bound traffic which resulted in congestion at peak times. The problem was made worse by the Barnes Dr intersection with congestion for Mosgiel bound traffic ‘‘on occasion'' stretching back to Anderson Bay Rd.
‘‘Retaining the current SH1 configuration in this area will result in significant pressure on the local network as motorists look to avoid the congestion on SH1 by using alternative routes.''
The note said the maximum length of queues on the Caversham bypass, for Mosgiel-bound, peak afternoon traffic, was 236.6m.
The peak traffic periods were ‘‘relatively short'' but resulted in ‘‘significant'' changes in journey time ‘‘with a variation of almost three minutes between the lowest and highest journey time in either direction during the rush-hour tidal movement''.
It estimated completing the whole project would reduce journey times by between 15% and 43% for city-bound traffic and by 15% and 46% for Mosgielbound traffic, with the largest savings during peak flow.
Opus considered the full upgrade (as opposed to doing the minimum) could produce a 2min 42sec time saving for Dunedin-bound morning traffic in 2021 and a 1min 55.7sec saving for Mosgiel-bound traffic in 2021.
Otago-Southland state highway manager Niclas Johansson explained a widely used formula for calculating economic benefits of reducing travel time: ‘‘Because you go through that section of road a bit quicker, you can do other stuff instead.''
