Critics of the cannabis legalisation bill are concerned about how drug driving will be controlled on New Zealand streets if the drug is legalised in next year's referendum.
A police sergeant in the US state of Oregon says officers use a combination of clinical testing and looking for cognitive deficits when determining impairment in drug drivers.
The state legalised the use and possession of recreational cannabis, for those over 21, in 2015.
Corin Dann speaks to Oregon State Police sergeant Evan Sether, who is a drug recognition expert.
