This debate made me curious about legal TCH levels and Google took me to this site...
Drug-impaired driving is a challenging issue for states, particularly as the laws and cultural acceptable around cannabis continue to evolve.
www.ghsa.org
It shows the legalization status and driving laws for every state. The results are all over the board. Of course in states where it is illegal to possess marijuana there is generally a zero tolerance for levels. The thing I find odd is that some states - like California - it is legal for all uses but they have no set limit for impaired driving. Apparently you can drive around like you are Cheech and Chong and not be charged.
Zaon Collins tested at 3 ng per se. I didn't look at every state but here are the limits for some of the well known states that have legalized it for recreational use.
Nevada - 2 ng per se
Oregon - no limit
Washington - 5 ng per se
California - no limit
Colorado- 5 - ng per se -
Edit: permissible interference (don't know what that means)
DC - no limit
Edit : here are the other states with limits.
Illinois - 5 ng per se
Montana - 5 ng per se (Starting in October)
Ohio - 2 ng per se but recreational use is illegal.