The Transport Agency has estimated 95 percent of the country's open roads should have their speed limit dropped from 100km/h to 60-80km/h.
The Agency's online risk assessment tool called Mega Maps has been published.
It calculates what the ideal speed limit should be using information such as crash history, road conditions and traffic volumes.
It is also recommending reducing speeds on many city street to 30-40km/h.
RNZ transport reporter Ben Strang joins Corin Dann with the details.