I would suspect that over time standards of what constitutes being intoxicated by cannabis or other drugs will be found, though it may be a test of a person's dexterity. I'm not sure if a chemical test is truly determinative, but the same is true with alcohol. In some people, less than the legal limit has them flying with alcohol, while others seem quite sober at well over the limit. In the case of cannabis, if a measurement was the key, it should be based on THC levels from what I've been led to understand, CBD doesn't effect judgement according to what I've been told. Of course there are hundreds of other components which generally haven't been discussed much, future findings may show that they have effects that are both positive and negative and may need to be considered.
When it comes to driving offenses, I believe officers need to have a reason for making a stop before questioning whether the driver is under the influence of anything. After an accident, I have no problem with drivers being tested and charged if tests show intoxication.
An adaptation of MYDX technology might prove to be the answer to whether you're over legal limits with both alcohol and drugs if standards which can be analyzed from your breath can be established.