One study found a field sobriety test to correctly classify impaired driving state due to THC 65-75% of the time [1]. The chance of a false positive seems too high to me given the harsh penalty of a DUI. Field sobriety tests have been shown to be ~91-94% reliable for alcohol impairment [2].<p>1 - <a href="http:&#x2F;&#x2F;www.sciencedirect.com&#x2F;science&#x2F;article&#x2F;pii&#x2F;S0379073804007674" rel="nofollow">http:&#x2F;&#x2F;www.sciencedirect.com&#x2F;science&#x2F;article&#x2F;pii&#x2F;S0379073804...</a>
2 - <a href="http:&#x2F;&#x2F;hfs.sagepub.com&#x2F;content&#x2F;48&#x2F;3&#x2F;608.short" rel="nofollow">http:&#x2F;&#x2F;hfs.sagepub.com&#x2F;content&#x2F;48&#x2F;3&#x2F;608.short</a>