I don't know how you could measure for benzos and get an accurate impression of how much, or if, a person is being affected in a way that would impact their driving ability.
First off, there are many people who at a certain dose are "normal", any less then that dosage then they are in withdrawals (which is bad for driving), any more then that they may be "intoxicated", so to speak (which is also bad for driving).
Secondly, because of tolerance you wouldn't be able to come up with a good benzo measurement that would indicate how incapacitated a person is. For some, 0.5mg Xanax knocks them out, while others 2mg Xanax has virtually no affect.
Thirdly, because of the large variance in benzo half-lives how are they going to tell how long ago the benzo was ingested, and whether it's still exerting an influence?
Lastly, like someone mentioned, many people need their benzo just to be able to function normally.
Sounds like a bad road to be going down.