An issue with having the legal limit at ~2-5ng/ml is that it makes habitual users be over the limit if they have smoked recently or not.[0] Making the prohibition seem unserious to some, not about safety but about punitive control, and in turn making it matter less if you smoke and drive as you are taking the risk of getting into trouble in any case.

The impairments of driving under the influence of alcohol have been extensively studied, but unless I have overlooked the literature it seems that the same investigations have not been carried out with THC.

[0] «Blood THC >2 ng/mL, and possibly even THC >5 ng/mL, does not necessarily represent recent use of cannabis in frequent cannabis users.»; https://www.sciencedirect.com/science/article/abs/pii/S03768...