The fact is that there is no level of THC in blood above which everyone is impaired, and below which no one is impaired. This is not because we need more studies; it’s because of human biology. Consider the following three factors.
Links
[1] https://docs.google.com/document/d/1x-_MN4txqJt6-wcn_-6Z-bgehntmvRj_MtS5Z_GiL3U/edit?usp=sharing
[2] http://www.rethinkpot.org/tags/wood
[3] http://www.rethinkpot.org/tags/potency
[4] http://www.rethinkpot.org/tags/2017-legislation
[5] http://www.rethinkpot.org/tags/thc-levels
[6] http://www.rethinkpot.org/tags/thc