Given that there's nothing resembling a "scientific" way to determine if one's "under the influence" of grass the only two options are to either accept the testimony of cops as fact *or* refrain from prosecuting any such case.
Neither option is very appealing.
Yeah, if the cop can’t confiscate the smoking joint, no prosecution will hold up in court.
Not for the foreseeable future.
And no appeals court will take the word of the cop alone.
Thank God.