A major problem, Gary, is that unlike with alcohol, the amount  of the substance in an individual's  system is not well correlated with actual impairment.  With alcohol, for example, .08 will indicate a a moderate level of impairment,  and twice that, .16, suggests serious impairment.
No such standardized  levels exist for cannabis.
Thus your breath test might indicate a high level of THC in a long-time user without noticeable actual impairment, while missing a  case of a new user seriously impaired by a small amount of THC.
Milburn's Druid test linked by MC has at least the potential for measuring intoxication rather than mere chemical presence.  It's a start towards common sense, fairness, and safety.
But first, let's check with law enforcement to see if there 's an actual increase in marijuana-related accidents--if it ain't broke don't fix it. 
 Who knows, if the party gang starts switching from booze to pot for their  weekend high, there could even be a reduction of accidents!