Let me start by saying that no one should operate a motor vehicle while impaired. If an accurate test can be developed to test for impairment by drugs, I'm all for it. Heck, I'd be happy if there was a roadside IQ test if it would keep morons off the road.
That said, there has been a great deal of misinformation and 'reefer madness' paranoia in this thread. Shazam - I'm mainly talking about you.
The attitude towards marijuana in Holland CLEARLY shows that permitting access to it LOWERS use of the herb. I guess if it's not forbidden, its not worth trying.
http://www.drugwarfacts.org/thenethe.htm
Use of marijuana is lower in Holland that the US. Drug related crime is less and there is clear evidence that pot isn't a gateway drug.