A recent study by the University of Sydney found little significance to THC levels in blood or saliva in predicting impairment of the individual. This was a meta-study which pooled data from 28 publications involving consumption of either ingested or inhaled forms of cannabis. Blood and oral fluid THC concentrations were measured. Driving performance skills were assessed.
For infrequent cannabis users, there appeared to be significant correlation between the THC concentrations and driving performance, but that relationship was weak.
For regular users of cannabis, there was no significant relationship between THC concentrations and driving performance.
The takeaway should be that while THC intoxication can definitely impair driving, THC concentrations from blood or saliva is not a way to determine THC intoxication.