The Automobile Association says the police need to carry out more roadside tests to stop drivers under the influence of drugs slipping through checkpoints.
Between November 2009 and the end of January, police did 580 impairment field tests for drugs.
Those who failed were made to take a blood test, and more than 500 drug charges were laid. To date, there have been 329 convictions.
The AA's motoring affairs general manager, Mike Noon, says not enough roadside testing is being done.
But the police say the number of field tests is small because police are testing for alcohol first.
The fewer than 600 roadside impairment tests for drugs since 2009 compares to the 8900 breath tests done for alcohol every day.
