Aucklanders will drive around 1 billion more km on the state highway network this year, 21 per cent more than a decade ago.

Congestion and safety have both been heading the wrong way, leaving many scratching their heads for an answer.

A quick look at road data and the reason is clear. Far from building too much road, we have built too little — or more precisely, our rapid population growth has outstripped our investment in roads.

In 2008, New Zealand had 175,569 lane km of road. We now have 179,079km. That's an increase of 2 per cent.

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However, New Zealand's population grew by 12.5 per cent over the same period. So we have many more people now per km of road than a decade ago.

More people doesn't necessarily mean more driving — especially given there's been a material increase in public transport investment.

A better metric then is "VKTs" — vehicle km travelled, or how far all New Zealand vehicles in total travelled each year.

This number increased 11.7 per cent between 2008 and 2017.

The numbers suggest a fraction of road was built as would be required to keep the ratio of vehicles to road length constant. More driving on relatively less road is what drives congestion and increases risks.

But this is only half the problem. In Auckland, population increased 11 per cent between 2007 and 2017 but under 4 per cent more lane km were built over this time.

Population growth has outstripped roading by a factor of almost 3.

Even worse, Aucklanders will drive around 1 billion more km on the state highway network this year, 21 per cent more than a decade ago.

Yet NZTA data shows state highway lane kilometres have increased less than 5 per cent over the same period (from 1177 in 2007 to 1,228 in 2017), despite investment in motorway improvements.

This is why Auckland is getting more and more congested, drivers more frustrated and roads less safe. It's not more roads that are the problem, it's more vehicles per km of road.

With growth at current levels we are heading to gridlock within a decade unless we deliver a step change in investment in public transport and roads and introduce road pricing sooner rather than later.