Rob,
Do you really believe that there are less people taking controlled substances in North Wales now than there was in 2009?
The comparison with parking fines is flawed, illegal parking is not an addictive activity> your comparison should be with tobacco and alcohol use and even that is difficult because use of tobacco and alcohol are lawful activities and the demand for them is controlled by taxation. You have no tools to control unlawful activity like drug use except enforcement or lack of it as the case appears to be here.