You probably see and swerve around them often, but have you ever wondered how potholes form in the first place? Or why some parts of New Zealand are more likely to have them? 1News fills in the gaps about these bumps in the country’s roads.

National has been waging war on potholes for some time.

The party launched a “pothole of the week” campaign back in 2022 and made big promises pre-election to set up a pothole repair fund.

Now that National is in Government, it’s allocated nearly $4 billion over three years to tackle the tens of thousands of potholes it says are blighting New Zealand’s highways.

But how did we wind up with so many potholes? And will the billions of dollars be enough to deal with the problem?

How potholes are created

James Smith from the National Road Carriers Association said he’s “very happy” with National’s $500m plan.

Potholes are formed at weak points in the road, usually as a result of breakdowns in the substrate, says AUT engineering professor John Tookey.

The substrate is what a road is built on and is usually a sequence of packed layers of graded stone. At the very base of the road are very large stones that are topped with smaller infill. This is all compacted down before tar and chipseal is added.

When you’re dealing with the mass production of something as large as a road, it’s not uncommon for voids or inclusions to create an abnormality or variation on the road’s surface, Tookey said.

“Then you can end up with stress concentrated in the surface around that abnormality,” he said.

“As the stress concentrates, it starts to create either a dip or depression or a lump … then every successive vehicle that goes over it creates further wear and further depression and further compression in that space and it gradually extends it out,” he said.

As the pothole begins to form, vehicles will often try to avoid it and thus drive along the edge of it, breaking that part of the road down further.

The weather factor

The weather can speed up the formation of potholes in several ways, too.

The most destructive form of pothole formation is associated with freezing temperatures, Tookey said.

“Water collects in holes [in the road] and if you have a freeze overnight, the water expands,” he said.

“The force that’s generated is tremendous, which means after a winter period in somewhere like the South Island you’ll end up getting all sorts of potholes formed as a result of that freeze-thaw cycle taking place.”

Extreme heat can also lead to potholes.

“If it’s very hot, the chipseal surface gets very plastic because it’s ductile and it can stretch,” Tookey said.

“Globs of it get pulled off by cars when it’s very hot and that creates further dents in the roadways that allow these wear holes to increase in size.”

Meanwhile, rain can create potholes in metal roads.

“You can end up getting heavy rain scouring tracks and consequently you get formation of potholes in back country roads.”

Some types of roads are less susceptible to potholes, such as those made with concrete.

“Concrete is rigid; it doesn’t give at all,” Tookey said. “When you cast concrete into the ground, you end up having a very rigid roadway, which tends not to pick up the same sort of wear effects that we get with our road surfaces.”

But there’s a catch – concrete roads are prohibitively expensive.

“They are not easy to put in place,” Tookey said. “They take a lot of time, and they are massively expensive.”

Maintaining the health of our highways

NZ's road maintenance burden is growing

The ductile surfaces of New Zealand’s roads mean they need maintenance more often than concrete roads – and keeping up with that maintenance is no easy feat, Tookey said.

“Every time we put in a new road or bypass or a new extension to this motorway, we add and add to our existing portfolio of roading that we have to maintain,” he said.

“We create something like Transmission Gully and that’s great … but we’re continuously adding to the burden of ongoing maintenance.”

The reality, Tookey said, is that New Zealand has a small population spread over a country with a lot of roads.

“Consequently, in terms of our personal tax liability to pay for the maintenance of roads, it’s disproportionate as a result.”

The costs associated with road maintenance are also huge. Tookey said even the eye-watering sums the Government is planning to spend on fixing the nation’s potholes will be eaten up quite quickly.

“The problem is that it fixes [the potholes in the roads] for a period of time and then what?” he said.

“It’s the proverbial greasing of the wheel; the grease needs to be applied continuously otherwise it’s going to start squeaking and will eventually drop off.

“Yes, a short-term commitment to try and get ahead of the issue is going to be worthwhile … it’s a good investment in road safety.

“The problem is that when you take your foot off the gas metaphorically and you go back to ‘normal’, well, normal is what got us to where we are now. You’ll fix the problem for two or three years, but the freeze-thaw cycle doesn’t go away, the weather doesn’t go away.”

The costs associated with road maintenance are huge

Green Party transport spokesperson Julie Anne Genter criticised the Government’s pothole repair fund earlier this year, saying long-term vision was needed.

“Pouring $4 billion into potholes might as well be money poured down the drain if we fail to confront the climate crisis and prevent a future filled with wave after wave of natural disaster,” she said.

“To plunge huge amounts of taxpayer money into something as short-term in its impact as fixing potholes is simply throwing good money after bad.”

But Transport Minister Simeon Brown said last month his pothole fund will deliver real results.

“Kiwis want potholes fixed and our roads properly maintained, and this funding boost will deliver real results across the country,” he said.

“Boosting investment in pothole prevention will deliver a safe and efficient network for New Zealanders that increases productivity and supports economic growth.”