Many New Zealand drivers will have felt frustration at the lowered speed limits in recent years.

But the lowering of speed limits doesn’t just save lives, it can also help to reduce emissions, says a New Zealand roading agency.

Waka Kotahi, as part of the outgoing government's Road to Zero safety campaign, estimates it has lowered limits from 100km/h to 80km/h on about four per cent of New Zealand highways.

This move has been one backed up by research. Experts say an individual's chances of surviving a head-on car crash decreases rapidly when the vehicles are travelling 80km/h or lower.

In an unintended consequence, lower speeds have also reduced emissions.

Waka Kotahi modelling shows the optimum speed for light vehicles travelling on New Zealand roads is 70-75km/h.

Anything above that increases the car's emissions impact.

"The faster you go the more the fuel is used to drive the car and overcome the wind resistance," says Ralph Sims, emeritus professor of sustainable energy and climate mitigation at Massey University.

He has been studying documents from Waka Kotahi.

These show the potential emissions increase from making the whole Waikato expressway 110km/h would be about five per cent over the next few years - even once the increasing electric vehicle fleet was accounted for, he says.

For any roads shifted from 80km/h to 100km/h, Waka Kotahi's model predicts an average increase of five per cent to 10 per cent of emissions.

This is calculated using the Vehicle Emissions Prediction Model developed by the transport agency and Auckland Council several years ago.

It predicts emissions from vehicles in New Zealand's fleet under typical road, traffic, and operating conditions.

Len Gillman, an expert in conservation biogeography, says these results aren't unique to New Zealand.

"The OECD has done work which shows a reduction from 110 to 90 kilometres per hour reduces emissions by 23 per cent. It makes a huge difference how fast you go."

Despite these benefits, the speed limit changes have frustrated many drivers - and the incoming National government has been vocal in its criticism.

Speaking to RNZ in September, National Party transport spokesperson Simeon Brown said they would be asking Waka Kotahi to review the recent speed limit decisions.

"They will have to take into account the benefit-cost analysis which looks at the impacts on travel times."

Brown expressed particular concern for the economic impacts of prolonged travel times.

But Gillman says the difference is actually very small.

"Dropping the maximum speed by 10 kilometres per hour only makes two to four minutes difference over 100 kilometres.

"It's only minimal - enough time to get in the queue for a coffee, but that's all."

According to Professor Sims, the sacrifice is worth it.

"Transport emissions are about 19 per cent of our total greenhouse gas emissions, and most of that is road transport.

"A typical family car in one year would produce around three tons of carbon dioxide. So that's a significant footprint - that's equivalent of flying from Auckland to the UK."

He says any move by the incoming government to reinstate faster speed limits will be a major blow to climate action.

"It goes against Climate Change Commission advice. It goes against our international commitments to reduce our national greenhouse gas emissions. And we need all the help we can get."

But will this be enough to get frustrated drivers on board?

Professor Sims says lowering speed limits will always be a hard sell to Kiwis.

"People in New Zealand love their cars, and they love to drive at fast speeds."

-Emma Ricketts/RNZ.