BAC, "objectively (though arbitrary) measurable impairment," and observable physical impairment are different things. 

People who try to drive while they are actually physically impaired due to marijuana will be caught the same way people who try to drive while physically impaired on any other substance are caught. Their driving creates reasonable suspicion for the police to pull them over and conduct a field interview. While there isn't yet a "breathalyzer" for pot, most people create the required probable cause for arrest, where a blood draw for pot can be conducted if necessary, long before the police have them blow. It's used to confirm what the police have already observed.