And by the way...no proof needed.
Certainly not for creating a new "sign" regulation.
As long as they're just guessing at "possiblys" and "could bes" and "mays", I think that those most concerned about this matter (who I'm guessing are likely 100% non-consumers of cannabis) should just really get it out there.
Hang giant posters of the William Randolph Hearst produced Reefer Madness posters in all the state legal dispensaries.
Show black men becoming homicidal after smoking pot and then raping white women.
Have posted warnings that tell males who smoke pot that they have a "possible" chance of growing female-sized breasts.
And require all dispensaries to have a video playing a non-stop loop of all the ONDCP-sponsored reefer madness television spots from the past 20 years. Make sure and include the one where the teenage boy smokes a joint, then picks up a loaded handgun off the kitchen table, points it at his friend's face and pulls the trigger.
It's all good. Just stop impeding adult Americans from having reasonable access to their desired medicines.
Which thankfully, is what's happening in California since 1996 and with increasing availability now that President Obama has ordered the DEA to cease and desist with harrassing medical marijuana dispensaries which comply with state laws and regulations.