Police say they cannot say precisely why it is on the increase. Although an increased number of checks may have contributed to the higher amounts, no explanation was found. In previous years, the majority of impaired drivers were found with alcohol only.

Now, it is drug-related offences — sometimes but not often with alcohol — which constitute the bulk of cases. Saliva is usually a screening test, with confirmatory testing done on blood.

Experts warn of long-lasting impairment
The Trimbo Instituut's drug researcher, Steven Biemans, comments: "Those using do so for two reasons: in the first place there are those convinced that they are better drivers when they are on drugs they know are having an impact, and also those thinking that they are free of any problems because they have stopped feeling any effects.

Drug effects last longer than perhaps many users appreciate, he cautioned.

"One dose of ecstasy can be retrieved in a test for up to 40 hours, multiple doses for more than two days," Biemans said. The gold standard is not driving at all is about a day for regular users.

He added that police testing devices are calibrated to detect this 'prolonged period of impairment'.

Police say doped driving has led to deadly accidents. Biemans stressed that the inconvenient testing has nothing to do with making things difficult for drivers — rather, the measures are to count.