Not my belief that driving with alcohol in the bloodstream is the main cause of accidents. In my humble view, excessive speed, inattention and unable to control a car properly are the major causes of fatal accidents, and yes I agree alcohol does contribute to a number of these.
As far as drug use is concerned, we all know it is unfortunately wide-spread in this country, but what bearing it has on one's driving I would not know, I have never taken drugs. I certainly like my drop of intoxicating liquor and do know the effect of this. I like to think I am a responsible driver because I know what over-indulgence can lead to, so have always drunk with caution, and knew when not to drive.
I agree RBT's certainly have altered peoples drinking and driving habits, however speed detection units (like radar and camera's) have also had a dramatic effect on speeding drivers and for me have certainly been more responsible for the lowering of the road toll than RBT's. Not forgetting road education either, much harder to obtain a licence these days then years ago, and publicity about accidents in the media.
Lezzles have a look at the following site and you can see the trend of fatal accidents in South Australia for the past 30 odd years. Certainly backs up your views in the early part when RBT's ,radar and camera's were first introduced, but the figures strangely enough haven't dropped over the last 12 years.
http://www.transport.sa.gov.au/rss/content/safer_people/issues/sa_crashes.htm