The law is fairly well set for alcohol, but for the other drugs it's a lot less so, and law enforcement generally operates on a fairly standard sobriety test as a precursor to more precise tests. Jazz Shaw takes a look at the problem in this article at Hot Air.
Drunk driving laws based on blood alcohol content (BAC) are sketchy enough as it is in my opinion. People react to alcohol differently for a number of reasons. Small, thin people will, theoretically, get drunk faster than larger folks and the BAC measurement allegedly takes that into account. But some habitual drinkers may seem essentially unimpaired at a BAC of .010 while someone who never drinks may be slurring their words at .008. There are also significant questions about the legality of making people take a sobriety test without a warrant, and these are being looked at by the Supreme Court this yearIt will be interesting to see what the Supremes have to say about the issue.
We need good science without a lot of hype on the issue of sobriety tests and driving while impaired. I haven't worked traffic or standard patrol since 2003, so I'm not totally current on best practices right now, but I know that during a traffic stop, nothing is ever cut-and-dried.