The business case that underlies the Auckland Transport and Government proposal for light rail from the Wynyard Quarter through Mt Roskill to Auckland Airport contains a number of features that may surprise people.

For example, it indicates that to minimise traffic interference and achieve the proposed 42-minute journey time from Britomart to the airport, trams will run on a kerb-protected central reservation limiting road traffic in each direction to a single lane shared between cycles, cars, buses and trucks, and restricting right turns into or out of Dominion Rd.

It will also limit Dominion Rd tram stops to 800m intervals, about twice the current bus stop distance.

Further, it shows the airport serves 14.5 million passengers a year, of whom approximately 20 per cent travel to or from the city centre. This equates to about 3 million passenger trips a year.

It also estimates 20,000 workers are employed within the airport precinct and presents information from a survey showing the areas from which about three-quarters of them commute to work.

The surveyed commuting workers make 6.6 million trips a year, more than twice as many as current airport passengers who might use the proposed light rail line. More than half the workers commute from South Auckland, Papakura and Manukau, areas that would not be served by it.

The business case tells us that at the proposed frequency of six trams an hour with double trams, it provides a capacity of 960 seated or 2520 total passengers an hour in each direction. This capacity might be doubled if the frequency were increased to 12 trams an hour, a theoretical maximum.

This is of course the total capacity of the line: not all of these passengers will be travelling the whole distance to or from the airport.

An alternative passenger transport solution for airport access, a rapid busway between Puhinui train station and the airport, is said in recent Government statements to be the "second priority" after the Dominion Rd light rail.

From Puhinui, Auckland commuter trains can carry 240 seated and 450 total passengers per three-car EMU, but at peak times six-car EMUs at 12 an hour provide for 5760 seated and 10,800 total passengers an hour in each direction. This is the current timetable, not a theoretical best-case performance.

With 12 50-seat shuttle buses an hour, the Puhinui-Airport capacity would be 600 passengers an hour in each direction. Its maximum theoretical capacity, with 50 shuttles an hour, would be 2500 passengers an hour in each direction - more than enough to provide for the 30-year airport passenger numbers projection.

The road distance between Puhinui Station and the airport along State Highway 20B is 6.8km (domestic terminal) and 7.3km (international), a distance that should be easily traversed within 10 minutes by a shuttle service on a dedicated busway that has a maximum speed of 90km/h.

To this add transfer and wait time at Puhinui (average three minutes or, worst-case, six minutes if there are 12 shuttles an hour) and the expected journey time from Britomart to the airport based on the current rail timetable, which shows 29-30 minutes from Britomart to Puhinui, is between 42 and 46 minutes. This is comparable with the business case's theoretical light rail journey time of 42-44 minutes.

The light rail proposal has not even begun to seek planning permission, and it would take at least two years to build. It would cause great disruption to traffic and businesses in Queen St and Dominion Rd during construction, and once in use would place ongoing severe restrictions on traffic and access to businesses in Dominion Rd.

Shuttle transport between Puhinui and the airport could in principle begin next week on the existing roadway, although the planning, design and construction of the busway and transfer stations to optimise its performance might take two years. The SH20B route is largely over open fields.

The business case estimates the cost of light rail to the airport at a total of $2.3 billion — $1.3b for Mt Roskill to the airport in addition to the $1b proposed for the Wynyard–Mt Roskill tramline.

The cost of the Puhinui-Airport shuttle busway with two stations is likely to be less than $300 million, given that the construction cost for the 6.2km northern busway and five stations was $294m.

In summary, the proposed light rail would cost $2b more than a Puhinui-airport shuttle for half the ride frequency, half to a quarter of the passenger capacity, a catchment that excludes half of the airport workers, and a theoretical journey time advantage of two minutes.

When we recognise dire problems in school and hospital buildings and the Prime Minister is warning, "We will not be able to address nine years of neglect in one Budget", which of these transport alternatives would taxpayers and ratepayers prefer?

• Ross Boswell is a pathologist and physician in the public hospital system. He lives in the central city and regularly uses public transport.