JohnD is quite correct, comparative increases or decreases do not prove that absolute numbers are greater or smaller.
When it rains, the puddle outside my house grows by 300%, whereas the nearby lake hardly seems to increase in size at all.
According to HWM logic the puddle must hold more water than the lake.
There are other factors to consider also:
drugs remain in the system considerably longer than alcohol.
Yes, many drugs stay in your system significantly longer than alcohol. While alcohol is typically metabolized and cleared from the body within 12–24 hours ..., many drugs and their metabolites can be detected for weeks or even months depending on the substance and the type of test.
A new campaign was launched in April 2025
So there will have been a parallel increase in random drug testing.
Whereas drink-driving has been under the microscope for decades, and has become socially unacceptable.
Thus "data" will be manipulated to support the measures to discourage drug-driving. (and clearly the data manipulation has worked on HWM and Pete who now think it's an absolute increase.).
The testing equipment for drugs is under constant development. So while there may not be an absolute increase in drug-driving, improved testing equipment might suggest an apparent increase.