Thanks mate, I have done some research as you suggested and the...

Thanks mate, I have done some research as you suggested and the US Marijuana Policy Project 2008* concludes that "Significant work is being done to develop and implement modified field sobriety tests, which measure the behavior of drivers (reaction time, for example) rather than their bodily fluids" and that this is a safer, more reliable and less legally problematical way to go.

*Marijuana and DUI Laws: How Can We Best Guard Against Impaired Driving?