Should Auckland spend billions of dollars on a "tram to the airport?" If that was really the question, then the answer would be "no". But that's like saying State Highway 1 is only for trips between the Bombay Hills and Wellsford.

Should Auckland build a new 23km rapid transit line, with vehicles carrying 500 people each, running at high frequencies, in dedicated lanes giving a fast, reliable journey? A line that links Auckland's biggest and fastest growing employment areas (the city centre and the airport), that passes by 10 per cent of the city's population and supports the growth of thousands of new homes from Mt Roskill to Mangere? A line that relieves increasing bus congestion in the city centre, and all for the same price as the 3.5km City Rail Link? Well, that's an entirely different question with an entirely different answer.

The light-rail project between the city centre and Auckland Airport needs to be built. But just because the line goes from the city to the airport doesn't mean it's only about those trips. In fact, it's not really about the airport much at all.

Light rail emerged as a serious idea in early 2015 to deal with Auckland's growing bus congestion problem. Auckland has been successful in increasing public transport ridership over the past 25 years - from 33 million annual boardings in the mid-1990s to 92 million today. Bus usage alone has more than doubled to 65 million and now more than 50 per cent of people entering the central city every morning do so not in a car. Without this, Auckland's huge population growth would have truly ground the city to a halt with over 200,000 extra trips on our roads daily.

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The growing use of buses, trains and ferries is starting to create problems of its own. The City Rail Link, now under construction, will allow us to run more trains. But our rail network does not serve the central isthmus, the southwest, the northwest or the north. Many of our key bus corridors in the city are already over capacity, or will be within a few years. Every morning around 140 buses an hour travel along Symonds St, more than the bus lanes can handle. As a result, buses regularly bunch up leading to delays for the thousands onboard. This problem will only get worse over time.

Light rail upgrades our busiest bus corridor, Dominion Rd, to modern light rail with a new route through the city. The project also upgrades Queen St and the town centres along Dominion Rd as high quality urban spaces. Taking the Dominion Rd buses off Symonds St frees up space to run more buses from other parts of the city. As an added bonus, it sets up the possibility of being extended to provide rail services to the North Shore.

There is nothing light about the capability of light rail. In Seattle, a similar system carries more people than our entire rail network.

The case for light rail on Dominion Rd to Mt Roskill is strong. The previous government recognised this and last year included it in their joint planning work with the council, after first investigating whether better buses could achieve the same thing (spoiler, they couldn't).

Which brings us back to the airport. The number of people travelling to and from the airport is growing quickly. Air passenger numbers will soon hit 20 million a year (up 5 million in the past 5 years). In the coming decades that is expected to reach 40 million. There are now regular horror stories of trips to and from the airport taking as long as flights, despite the completion of road projects intended to do the opposite. But the airport alone is not worth building light rail for, especially given that a fast bus link to a rebuilt Puhinui train station will be running by late 2020. In fact, detailed transport modelling suggests that barely 10 per cent of people using this line will be heading to the airport for their flights.

Instead, the Mt Roskill to airport section of the light-rail system is about connecting the people who work in and around the airport. It's about the people who live around Mangere, and have never had good public transport to the airport or the wider city. And it's about supporting a transformational growth programme across a swathe of Auckland.

There are huge tracts of Housing New Zealand land in Mt Roskill and Mangere that will become prime redevelopment opportunities with light rail in place. There's also Onehunga, perhaps Auckland's most complete suburban town centre with massive growth potential. Light rail shouldn't, and can't, just be a transport project. It must also be a growth project.

Matt Lowrie is the director at urban advocacy group Greater Auckland