Roadside cameras which snap vehicles at two points along the same stretch of road to calculate if they have been speeding, may be used on New Zealand roads.

The idea was dropped two years ago by the previous Government and there are concerns the cameras will be a revenue-grabbing tool, and could be used to spy on motorists.

But Associate Transport Minister Julie Anne Genter has sought advice from the Ministry of Transport on a raft of measures to reduce speed on dangerous roads, including the use of point-to-point cameras, or section control.

Other measures include simplifying the process for creating bylaws for speeding, mandatory devices on heavy vehicles to prevent cyclists and cars from sliding underneath them in crashes, and allowing cyclists to ride on the pavement in some circumstances.

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Genter said the cameras had proved "extremely successful" in Australia, the UK and Europe.

"We are currently considering how they can be rolled out in a fair and effective way here in New Zealand," she told the Herald on Sunday.

Genter said work was still being done on where the cameras might be installed and how much they would cost.

"Obviously we want to target the highest-risk corridors because we want to reduce our horrific and increasing rates of deaths and serious injuries on the road."

The cameras work by taking a photo of every vehicle entering a specified stretch of road, such as an accident black-spot, then snaps them again at the end. The average speed is then calculated by dividing the distance travelled by the time taken to get there. If it is higher than the speed limit, an infringement notice is issued.

"They're no different to the type of safety cameras that are currently used. It's just that it gives a more accurate reading of average speed in a particular area rather than just a snapshot in one place," Genter said.

The previous National Government, which promoted the cameras in its Safer Journeys 2010-2020 road safety strategy, considered them at Cabinet level in 2016 but did not implement them.

National's associate transport spokesman, Brett Hudson, was against the cameras "for a host of reasons".

"They'll give you a snapshot but it tells you absolutely nothing about the driver's behaviour in between. You don't capture road safety behaviour at all. At least with speed cameras, at a specific point you do.

"The public would simply perceive it's all about revenue-grabbing because there's just no direct link with the behaviour in between."

Genter rejected the suggestion.

"Yes, revenue is a result of us having infringement fines, but arguably the best case scenario is that people stop travelling at unsafe speeds and we have fewer serious injuries and deaths."

Hudson also suggested the cameras could be used to spy on motorists, because they took images of all vehicles - instead of just those speeding - at a particular time, at a particular place.

"We will be surveilling New Zealand citizens. I think people should feel a bit uncomfortable about that."

Mark Stockdale, the Automobile Association's regulations spokesman, said informing the public about the cameras' use was key to acceptance of the technology.

"What the AA would like to see is policy work in relation to cameras so that motorists can see the reason why point-to-point cameras are being proposed or installed.

"You have to get the public on board. You have to have the conversation with the public about why point-to-point cameras are going to contribute to reducing our road toll and accidents, particularly where speed is a factor."

Stockdale said the public would also have to be convinced that the cameras were the right investment to improve road safety.

"Are there other technologies that could be invested in that might be more effective? We know police resourcing is stretched and obviously these devices cost money. The AA wouldn't want to see the investment in these at the expense of better technology and better use of police resourcing."

Inspector Peter McKennie, Police road policing operations manager, declined to comment.

Road safety campaigner Clive Matthew-Wilson, editor of dogandlemon.com, questioned the cameras' effectiveness.

"Speed cameras alienate ordinary motorists without affecting the behaviour of the tiny minority who cause most fatal crashes."

But road safety charity Brake said the cameras had been shown to be effective at reducing the number of deaths and serious injuries on roads.

An initial $22.5 million programme of work to improve safety on 30 regional state highways is almost complete. That funding is in addition to an existing $100m per annum rural road safety improvement programme.

How the cameras work
Time-stamped images of each vehicle are taken on a stretch of road, one as they enter and another as they exit.

The vehicles are identified using number plate recognition software.

Software determines if the vehicle was speeding by dividing the distance between the start and finish points by the time taken to travel the distance.

A fine is sent to the motorist.

Overseas study
A study this year by the International Transport Forum - the OECD's international transport think-tank - said section control seemed very effective at reducing speed but noted a study on Italian motorways that effectiveness decreased over time.

Section control was introduced on Italian motorways in 2005. The study said the speed reduction was 45 per cent for vehicles exceeding the speed limit and 84 per cent for vehicles exceeding the speed limit by more than 20km/h.

However, the study also noted that the safety effectiveness of the system decreased over time, with crash reduction and speed reduction steadily declining in the three years after it was introduced.