When considering this issue, and clearly with a referendum on Cannabis pending I can both understand the sincere interest but journalistic balance must consider the facts as well as the emotional messageThe graphic [above]  puts the scale of the problem domain into context. We are talking up drug testing without the required policy impact statements.  Only from this base can we determine if the policy enforcement is effective. If Cannabis and Driving went up tenfold, it would make a negligible difference to the 'Odds' of being in drug-impaired culpable accident. Alcohol, it is clear, to quote Prof Doug Sellman, no ordinary commodity.... 

But if Cannabis displaces Alcohol = less grief all around.