The Transport Agency has estimated that 87 per cent of speed limits on New Zealand roads are too high.

The agency's online risk assessment tool, Mega Maps, uses a range of factors such as crash history, road conditions, surrounding land use and traffic volumes to calculate an ideal speed.

Mega Maps suggests only 5 percent of the open road should have the current 100 kilometre an hour speed limit, and in most cases a speed of 60-80 km/h should apply.

For most urban areas, Mega Maps suggests the safe and appropriate speed would be 30-40 km/h

The Mega Maps tool is used by NZTA and councils as a guide for deciding on new speed limits.

Road Transport Forum chief executive Nick Legget says with the government wanting to crack down on the road toll, he's concerned speed limits could drop across the country.

Nick says that would severely restrict the movement of freight and the productivity of New Zealand's economy, as well as having a major impact on the everyday lives of drivers.

Last month, Transport Minister Julie Anne Genter said the government's Vision Zero road safety plan seeks to drop the number of road deaths by half in the next decade and one tool was to reduce speed limits.

"We wouldn't have a blanket reduction.

"What is important is that we're going to focus on the most dangerous roads, revising them to have a safe and appropriate speed limit, and making it easier for local councils to get a safe and appropriate speed limit."

Nick says the suggestion of lowering speed limits was about slowing New Zealand down and potentially making it a lot less productive.

"The speed limit is 90 for trucks, and the worry for us is you make that 60 you are making it much much harder for the New Zealand economy to be productive for people to be able to move their goods around the country - and the economy will suffer."

New Zealand's population was too small to be able to subsidise public transport as other countries do, so it relied on roads.

"What we've seen in the last couple of years is a de-funding of the roading budget."

He says speed is one factor in safety - and backed lowering the limit on dangerous spots - but said New Zealand should prioritise improving the quality of the roads.